fix: write CLI and streaming errors to stderr

## CHANGES
- Route CLI execution errors to standard error output
- Print Anthropic stream errors to stderr consistently
- Add os import to support stderr error writes
- Preserve help-output suppression and exit behavior

cmd/fabric/main.go

@@ -12,7 +12,7 @@ import (
 func main() {
 	err := cli.Cli(version)
 	if err != nil && !flags.WroteHelp(err) {
-		fmt.Printf("%s\n", err)
+		fmt.Fprintf(os.Stderr, "%s\n", err)
 		os.Exit(1)
 	}
 }

internal/plugins/ai/anthropic/anthropic.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"os"
 	"strconv"
 	"strings"
 
@@ -216,7 +217,7 @@ func (an *Client) SendStream(
 	}
 
 	if stream.Err() != nil {
-		fmt.Printf("Messages stream error: %v\n", stream.Err())
+		fmt.Fprintf(os.Stderr, "Messages stream error: %v\n", stream.Err())
 	}
 	close(channel)
 	return