refactor: improve merge commit detection and update error messages

## CHANGES

- Move merge patterns to package-level variables
- Update date parsing error message for clarity
- Simplify author email field comment
- Extract regex compilation from function scope
- Improve merge commit detection performance
- Clarify RFC3339 fallback error context

cmd/generate_changelog/internal/cache/cache.go

@@ -207,7 +207,7 @@ func (c *Cache) GetVersions() (map[string]*git.Version, error) {
 			if err != nil {
 				v.Date, err = time.Parse(time.RFC3339, dateStr.String)
 				if err != nil {
-					fmt.Fprintf(os.Stderr, "Error parsing date '%s' with RFC3339 for version '%s': %v\n", dateStr.String, v.Name, err)
+					fmt.Fprintf(os.Stderr, "Error parsing date '%s' with both RFC3339Nano and RFC3339 for version '%s': %v\n", dateStr.String, v.Name, err)
 				}
 			}
 		}

cmd/generate_changelog/internal/changelog/processing.go

@@ -14,6 +14,14 @@ import (
 	"github.com/danielmiessler/fabric/cmd/generate_changelog/internal/github"
 )
 
+var mergePatterns = []*regexp.Regexp{
+	regexp.MustCompile(`^Merge pull request #\d+`),      // "Merge pull request #123 from..."
+	regexp.MustCompile(`^Merge branch '.*' into .*`),    // "Merge branch 'feature' into main"
+	regexp.MustCompile(`^Merge remote-tracking branch`), // "Merge remote-tracking branch..."
+	regexp.MustCompile(`^Merge '.*' into .*`),           // "Merge 'feature' into main"
+	regexp.MustCompile(`^Merge .*`),                     // General "Merge ..." patterns
+}
+
 // isMergeCommit determines if a commit is a merge commit.
 //
 // This function uses two methods to detect merge commits:
@@ -30,16 +38,6 @@ func isMergeCommit(commit github.PRCommit) bool {
 	}
 
 	// Fallback method: Check commit message patterns
-	// Common merge commit message patterns
-	// Pre-compiled regular expressions for common merge commit message patterns
-	mergePatterns := []*regexp.Regexp{
-		regexp.MustCompile(`^Merge pull request #\d+`),      // "Merge pull request #123 from..."
-		regexp.MustCompile(`^Merge branch '.*' into .*`),    // "Merge branch 'feature' into main"
-		regexp.MustCompile(`^Merge remote-tracking branch`), // "Merge remote-tracking branch..."
-		regexp.MustCompile(`^Merge '.*' into .*`),           // "Merge 'feature' into main"
-		regexp.MustCompile(`^Merge .*`),                     // General "Merge ..." patterns
-	}
-
 	for _, pattern := range mergePatterns {
 		if pattern.MatchString(commit.Message) {
 			return true

cmd/generate_changelog/internal/github/types.go

@@ -26,7 +26,7 @@ type PRCommit struct {
 	SHA     string
 	Message string
 	Author  string
-	Email   string    // Email of the author from GitHub API, empty if not provided by the API (e.g., when the author has not made their email public)
+	Email   string    // Author email from GitHub API, empty if not public
 	Date    time.Time // Timestamp field
 	Parents []string  // Parent commits (for merge detection)
 }