feat(music): friendly cross-platform ffmpeg-missing error

internal/music/music.go

@@ -1,6 +1,7 @@
 package music
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -104,7 +105,7 @@ func ResolveFFmpegWithConfig(configuredPath string) (string, error) {
 	}
 	path, err := exec.LookPath("ffmpeg")
 	if err != nil {
-		return "", err
+		return "", ffmpegMissingErr("ffmpeg")
 	}
 	return path, nil
 }
@@ -122,11 +123,23 @@ func ResolveFFprobeWithConfig(configuredPath string) (string, error) {
 	}
 	path, err := exec.LookPath("ffprobe")
 	if err != nil {
-		return "", err
+		return "", ffmpegMissingErr("ffprobe")
 	}
 	return path, nil
 }
 
+// ffmpegMissingErr renders an actionable, cross-platform "tool not found"
+// message. Only the music builder needs ffmpeg/ffprobe; every other builder
+// runs with zero external dependencies.
+func ffmpegMissingErr(tool string) error {
+	return fmt.Errorf(`%[1]s not found on PATH.
+  Windows: winget install Gyan.FFmpeg
+  macOS:   brew install ffmpeg
+  Linux:   apt install ffmpeg   (or your distro's package manager)
+Only the music builder needs %[1]s; set SOW_%[2]s to a full path to override.`,
+		tool, strings.ToUpper(tool))
+}
+
 func IsMusicAssetPath(rel string) bool {
 	rel = filepath.ToSlash(strings.TrimSpace(rel))
 	return rel == "envi/music" || strings.HasPrefix(rel, "envi/music/")