modernize: update GitHub Actions and modernize Go code with latest stdlib features

## CHANGES

- Upgrade GitHub Actions to latest versions (v6, v21)
- Add modernization check step in CI workflow
- Replace strings manipulation with `strings.CutPrefix` and `strings.CutSuffix`
- Replace manual loops with `slices.Contains` for validation
- Use `strings.SplitSeq` for iterator-based string splitting
- Replace `bytes.TrimPrefix` with `bytes.CutPrefix` for clarity
- Use `strings.Builder` instead of string concatenation
- Replace `fmt.Sprintf` with `fmt.Appendf` for efficiency
- Simplify padding calculation with `max` builtin

internal/cli/flags.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -230,8 +231,8 @@ func parseDebugLevel(args []string) int {
 			if lvl, err := strconv.Atoi(args[i+1]); err == nil {
 				return lvl
 			}
-		} else if strings.HasPrefix(arg, "--debug=") {
-			if lvl, err := strconv.Atoi(strings.TrimPrefix(arg, "--debug=")); err == nil {
+		} else if after, ok := strings.CutPrefix(arg, "--debug="); ok {
+			if lvl, err := strconv.Atoi(after); err == nil {
 				return lvl
 			}
 		}
@@ -241,8 +242,8 @@ func parseDebugLevel(args []string) int {
 
 func extractFlag(arg string) string {
 	var flag string
-	if strings.HasPrefix(arg, "--") {
-		flag = strings.TrimPrefix(arg, "--")
+	if after, ok := strings.CutPrefix(arg, "--"); ok {
+		flag = after
 		if i := strings.Index(flag, "="); i > 0 {
 			flag = flag[:i]
 		}
@@ -348,10 +349,8 @@ func validateImageFile(imagePath string) error {
 	ext := strings.ToLower(filepath.Ext(imagePath))
 	validExtensions := []string{".png", ".jpeg", ".jpg", ".webp"}
 
-	for _, validExt := range validExtensions {
-		if ext == validExt {
-			return nil // Valid extension found
-		}
+	if slices.Contains(validExtensions, ext) {
+		return nil // Valid extension found
 	}
 
 	return fmt.Errorf("%s", fmt.Sprintf(i18n.T("invalid_image_file_extension"), ext))
@@ -370,13 +369,7 @@ func validateImageParameters(imagePath, size, quality, background string, compre
 	// Validate size
 	if size != "" {
 		validSizes := []string{"1024x1024", "1536x1024", "1024x1536", "auto"}
-		valid := false
-		for _, validSize := range validSizes {
-			if size == validSize {
-				valid = true
-				break
-			}
-		}
+		valid := slices.Contains(validSizes, size)
 		if !valid {
 			return fmt.Errorf("%s", fmt.Sprintf(i18n.T("invalid_image_size"), size))
 		}
@@ -385,13 +378,7 @@ func validateImageParameters(imagePath, size, quality, background string, compre
 	// Validate quality
 	if quality != "" {
 		validQualities := []string{"low", "medium", "high", "auto"}
-		valid := false
-		for _, validQuality := range validQualities {
-			if quality == validQuality {
-				valid = true
-				break
-			}
-		}
+		valid := slices.Contains(validQualities, quality)
 		if !valid {
 			return fmt.Errorf("%s", fmt.Sprintf(i18n.T("invalid_image_quality"), quality))
 		}
@@ -400,13 +387,7 @@ func validateImageParameters(imagePath, size, quality, background string, compre
 	// Validate background
 	if background != "" {
 		validBackgrounds := []string{"opaque", "transparent"}
-		valid := false
-		for _, validBackground := range validBackgrounds {
-			if background == validBackground {
-				valid = true
-				break
-			}
-		}
+		valid := slices.Contains(validBackgrounds, background)
 		if !valid {
 			return fmt.Errorf("%s", fmt.Sprintf(i18n.T("invalid_image_background"), background))
 		}

internal/cli/help.go

@@ -183,10 +183,10 @@ func detectLanguageFromArgs() string {
 			if i+1 < len(args) {
 				return args[i+1]
 			}
-		} else if strings.HasPrefix(arg, "--language=") {
-			return strings.TrimPrefix(arg, "--language=")
-		} else if strings.HasPrefix(arg, "-g=") {
-			return strings.TrimPrefix(arg, "-g=")
+		} else if after, ok := strings.CutPrefix(arg, "--language="); ok {
+			return after
+		} else if after, ok := strings.CutPrefix(arg, "-g="); ok {
+			return after
 		} else if runtime.GOOS == "windows" && strings.HasPrefix(arg, "/g:") {
 			return strings.TrimPrefix(arg, "/g:")
 		} else if runtime.GOOS == "windows" && strings.HasPrefix(arg, "/g=") {
@@ -272,10 +272,7 @@ func (h *TranslatedHelpWriter) writeAllFlags() {
 
 		// Pad to align descriptions
 		flagStr := flagLine.String()
-		padding := 34 - len(flagStr)
-		if padding < 2 {
-			padding = 2
-		}
+		padding := max(34-len(flagStr), 2)
 
 		fmt.Fprintf(h.writer, "%s%s%s", flagStr, strings.Repeat(" ", padding), description)
 

internal/cli/output.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 
 	"github.com/atotto/clipboard"
@@ -66,10 +67,5 @@ func CreateAudioOutputFile(audioData []byte, fileName string) (err error) {
 func IsAudioFormat(fileName string) bool {
 	ext := strings.ToLower(filepath.Ext(fileName))
 	audioExts := []string{".wav", ".mp3", ".m4a", ".aac", ".ogg", ".flac"}
-	for _, audioExt := range audioExts {
-		if ext == audioExt {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(audioExts, ext)
 }

internal/domain/file_manager.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 )
 
@@ -146,14 +147,7 @@ func fixInvalidEscapes(jsonStr string) string {
 		// Check for escape sequences only inside strings
 		if inQuotes && ch == '\\' && i+1 < len(jsonStr) {
 			nextChar := jsonStr[i+1]
-			isValid := false
-
-			for _, validEscape := range validEscapes {
-				if nextChar == validEscape {
-					isValid = true
-					break
-				}
-			}
+			isValid := slices.Contains(validEscapes, nextChar)
 
 			if !isValid {
 				// Invalid escape sequence - add an extra backslash

internal/plugins/ai/gemini/voices.go

@@ -3,6 +3,7 @@ package gemini
 import (
 	"fmt"
 	"sort"
+	"strings"
 )
 
 // GeminiVoice represents a Gemini TTS voice with its characteristics
@@ -126,16 +127,17 @@ func ListGeminiVoices(shellCompleteMode bool) string {
 	if shellCompleteMode {
 		// For shell completion, just return voice names
 		names := GetGeminiVoiceNames()
-		result := ""
+		var result strings.Builder
 		for _, name := range names {
-			result += name + "\n"
+			result.WriteString(name + "\n")
 		}
-		return result
+		return result.String()
 	}
 
 	// For human-readable output
 	voices := GetGeminiVoices()
-	result := "Available Gemini Text-to-Speech voices:\n\n"
+	var result strings.Builder
+	result.WriteString("Available Gemini Text-to-Speech voices:\n\n")
 
 	// Group by characteristics for better readability
 	groups := map[string][]GeminiVoice{
@@ -186,22 +188,22 @@ func ListGeminiVoices(shellCompleteMode bool) string {
 	// Output grouped voices
 	for groupName, groupVoices := range groups {
 		if len(groupVoices) > 0 {
-			result += fmt.Sprintf("%s:\n", groupName)
+			result.WriteString(fmt.Sprintf("%s:\n", groupName))
 			for _, voice := range groupVoices {
 				defaultStr := ""
 				if voice.Name == "Kore" {
 					defaultStr = " (default)"
 				}
-				result += fmt.Sprintf("  %-15s - %s%s\n", voice.Name, voice.Description, defaultStr)
+				result.WriteString(fmt.Sprintf("  %-15s - %s%s\n", voice.Name, voice.Description, defaultStr))
 			}
-			result += "\n"
+			result.WriteString("\n")
 		}
 	}
 
-	result += "Use --voice <voice_name> to select a specific voice.\n"
-	result += "Example: fabric --voice Charon -m gemini-2.5-flash-preview-tts -o output.wav \"Hello world\"\n"
+	result.WriteString("Use --voice <voice_name> to select a specific voice.\n")
+	result.WriteString("Example: fabric --voice Charon -m gemini-2.5-flash-preview-tts -o output.wav \"Hello world\"\n")
 
-	return result
+	return result.String()
 }
 
 // NOTE: This implementation maintains a curated list based on official Google documentation.

internal/plugins/ai/lmstudio/lmstudio.go

@@ -140,8 +140,8 @@ func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.Cha
 			continue
 		}
 
-		if bytes.HasPrefix(line, []byte("data: ")) {
-			line = bytes.TrimPrefix(line, []byte("data: "))
+		if after, ok := bytes.CutPrefix(line, []byte("data: ")); ok {
+			line = after
 		}
 
 		if string(line) == "[DONE]" {

internal/plugins/ai/openai/openai_image.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 
 	"github.com/danielmiessler/fabric/internal/domain"
@@ -31,12 +32,7 @@ var ImageGenerationSupportedModels = []string{
 
 // supportsImageGeneration checks if the given model supports the image_generation tool
 func supportsImageGeneration(model string) bool {
-	for _, supportedModel := range ImageGenerationSupportedModels {
-		if model == supportedModel {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(ImageGenerationSupportedModels, model)
 }
 
 // getOutputFormatFromExtension determines the API output format based on file extension

internal/plugins/ai/perplexity/perplexity.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"strings"
 	"sync"
 
 	"github.com/danielmiessler/fabric/internal/domain"
@@ -107,18 +108,19 @@ func (c *Client) Send(ctx context.Context, msgs []*chat.ChatCompletionMessage, o
 		return "", fmt.Errorf("perplexity API request failed: %w", err) // Corrected capitalization
 	}
 
-	content := resp.GetLastContent()
+	var content strings.Builder
+	content.WriteString(resp.GetLastContent())
 
 	// Append citations if available
 	citations := resp.GetCitations()
 	if len(citations) > 0 {
-		content += "\n\n# CITATIONS\n\n"
+		content.WriteString("\n\n# CITATIONS\n\n")
 		for i, citation := range citations {
-			content += fmt.Sprintf("- [%d] %s\n", i+1, citation)
+			content.WriteString(fmt.Sprintf("- [%d] %s\n", i+1, citation))
 		}
 	}
 
-	return content, nil
+	return content.String(), nil
 }
 
 func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.ChatOptions, channel chan string) error {

internal/plugins/template/extension_executor_test.go

@@ -189,7 +189,8 @@ esac`
 	// Helper function to create and register extension
 	createExtension := func(name, opName, cmdTemplate string, config map[string]any) error {
 		configPath := filepath.Join(tmpDir, name+".yaml")
-		configContent := `name: ` + name + `
+		var configContent strings.Builder
+		configContent.WriteString(`name: ` + name + `
 executable: ` + testScript + `
 type: executable
 timeout: 30s
@@ -199,14 +200,14 @@ operations:
 config:
   output:
     method: file
-    file_config:`
+    file_config:`)
 
 		// Add config options
 		for k, v := range config {
-			configContent += "\n      " + k + ": " + strings.TrimSpace(v.(string))
+			configContent.WriteString("\n      " + k + ": " + strings.TrimSpace(v.(string)))
 		}
 
-		if err := os.WriteFile(configPath, []byte(configContent), 0644); err != nil {
+		if err := os.WriteFile(configPath, []byte(configContent.String()), 0644); err != nil {
 			return err
 		}
 

internal/server/ollama.go

@@ -102,7 +102,7 @@ func ServeOllama(registry *core.PluginRegistry, address string, version string)
 	// Ollama Endpoints
 	r.GET("/api/tags", typeConversion.ollamaTags)
 	r.GET("/api/version", func(c *gin.Context) {
-		c.Data(200, "application/json", []byte(fmt.Sprintf("{\"%s\"}", version)))
+		c.Data(200, "application/json", fmt.Appendf(nil, "{\"%s\"}", version))
 	})
 	r.POST("/api/chat", typeConversion.ollamaChat)
 
@@ -224,7 +224,7 @@ func (f APIConvert) ollamaChat(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "testing endpoint"})
 		return
 	}
-	for _, word := range strings.Split(fabricResponse.Content, " ") {
+	for word := range strings.SplitSeq(fabricResponse.Content, " ") {
 		forwardedResponse = OllamaResponse{
 			Model:     "",
 			CreatedAt: "",