feat(core,cli): implement track command with go-git staging

internal/homesick/core/core.go

@@ -213,8 +213,125 @@ func (a *App) UnlinkCastle(castle string) error {
 	return nil
 }
 
-func (a *App) Track(string, string) error {
-	return errors.New("track is not implemented in Go yet")
+func (a *App) Track(filePath string, castle string) error {
+	return a.TrackPath(filePath, castle)
+}
+
+func (a *App) TrackPath(filePath string, castle string) error {
+	if strings.TrimSpace(castle) == "" {
+		castle = "dotfiles"
+	}
+
+	trimmedFile := strings.TrimSpace(filePath)
+	if trimmedFile == "" {
+		return errors.New("track requires FILE")
+	}
+
+	castleRoot := filepath.Join(a.ReposDir, castle)
+	castleHome := filepath.Join(castleRoot, "home")
+	info, err := os.Stat(castleHome)
+	if err != nil || !info.IsDir() {
+		return fmt.Errorf("could not track %s, expected %s to exist and contain dotfiles", castle, castleHome)
+	}
+
+	absolutePath, err := filepath.Abs(strings.TrimRight(trimmedFile, string(filepath.Separator)))
+	if err != nil {
+		return err
+	}
+	if _, err := os.Lstat(absolutePath); err != nil {
+		return err
+	}
+
+	relativeDir, err := filepath.Rel(a.HomeDir, filepath.Dir(absolutePath))
+	if err != nil {
+		return err
+	}
+	if relativeDir == ".." || strings.HasPrefix(relativeDir, ".."+string(filepath.Separator)) {
+		return fmt.Errorf("track requires file under %s", a.HomeDir)
+	}
+
+	castleTargetDir := filepath.Join(castleHome, relativeDir)
+	if relativeDir == "." {
+		castleTargetDir = castleHome
+	}
+	if err := os.MkdirAll(castleTargetDir, 0o755); err != nil {
+		return err
+	}
+
+	trackedPath := filepath.Join(castleTargetDir, filepath.Base(absolutePath))
+	if _, err := os.Lstat(trackedPath); err == nil {
+		return fmt.Errorf("%s already exists", trackedPath)
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+
+	if err := os.Rename(absolutePath, trackedPath); err != nil {
+		return err
+	}
+
+	subdirChanged := false
+	if relativeDir != "." {
+		subdirPath := filepath.Join(castleRoot, ".homesick_subdir")
+		subdirChanged, err = appendUniqueSubdir(subdirPath, relativeDir)
+		if err != nil {
+			return err
+		}
+	}
+
+	if err := a.linkPath(trackedPath, absolutePath); err != nil {
+		return err
+	}
+
+	repo, err := git.PlainOpen(castleRoot)
+	if err != nil {
+		return err
+	}
+	worktree, err := repo.Worktree()
+	if err != nil {
+		return err
+	}
+
+	trackedRelativePath := filepath.Join("home", relativeDir, filepath.Base(absolutePath))
+	if relativeDir == "." {
+		trackedRelativePath = filepath.Join("home", filepath.Base(absolutePath))
+	}
+	if _, err := worktree.Add(filepath.ToSlash(filepath.Clean(trackedRelativePath))); err != nil {
+		return err
+	}
+
+	if subdirChanged {
+		if _, err := worktree.Add(".homesick_subdir"); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func appendUniqueSubdir(path string, subdir string) (bool, error) {
+	existing, err := readSubdirs(path)
+	if err != nil {
+		return false, err
+	}
+
+	cleanSubdir := filepath.Clean(subdir)
+	for _, line := range existing {
+		if filepath.Clean(line) == cleanSubdir {
+			return false, nil
+		}
+	}
+
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return false, err
+	}
+	defer file.Close()
+
+	if _, err := file.WriteString(cleanSubdir + "\n"); err != nil {
+		return false, err
+	}
+
+	return true, nil
 }
 
 func (a *App) linkEach(castleHome string, baseDir string, subdirs []string) error {
@@ -420,9 +537,7 @@ func deriveDestination(uri string) string {
 	candidate = strings.TrimPrefix(candidate, "http://github.com/")
 	candidate = strings.TrimPrefix(candidate, "git://github.com/")
 
-	if strings.HasPrefix(candidate, "file://") {
-		candidate = strings.TrimPrefix(candidate, "file://")
-	}
+	candidate = strings.TrimPrefix(candidate, "file://")
 
 	candidate = strings.TrimSuffix(candidate, ".git")
 	candidate = strings.TrimSuffix(candidate, "/")