feat: initial gosleep-timer implementation

Go TUI sleep timer with modular architecture.
- Config: YAML-based, profile inheritance, auto WM detection
- Engine: timer, executor, stage pipeline (pre→timer→post)
- Modules: workspace (niri/hyprland/kde), media, lock, kill,
  notify, sound, brightness, mute, custom, script
- TUI: bubbletea app with profiles, module toggles, progress bar
- CLI: run, list-profiles, export, import, history, stats
- History: JSONL log with statistics
- QR: config export as ASCII QR

modules/sound/sound.go

@@ -0,0 +1,74 @@
+package sound
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type SoundModule struct {
+	player string
+}
+
+func (m *SoundModule) Name() string {
+	return "sound"
+}
+
+func (m *SoundModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Sound != nil && cfg.Sound.Enabled && cfg.Sound.File != ""
+}
+
+func (m *SoundModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	if stage != modules.StagePost {
+		return nil
+	}
+
+	file := ctx.Config.Sound.File
+	if file == "" {
+		return nil
+	}
+
+	player := m.player
+	if player == "" {
+		player = detectPlayer()
+	}
+
+	switch player {
+	case "paplay":
+		return []string{fmt.Sprintf("paplay %s", file)}
+	case "aplay":
+		return []string{fmt.Sprintf("aplay %s", file)}
+	case "ffplay":
+		return []string{fmt.Sprintf("ffplay -nodisp -autoexit %s", file)}
+	default:
+		return nil
+	}
+}
+
+func (m *SoundModule) Validate() error {
+	player := detectPlayer()
+	if player == "" {
+		return fmt.Errorf("no audio player found (paplay, aplay, or ffplay)")
+	}
+	m.player = player
+	return nil
+}
+
+func detectPlayer() string {
+	if _, err := exec.LookPath("paplay"); err == nil {
+		return "paplay"
+	}
+	if _, err := exec.LookPath("aplay"); err == nil {
+		return "aplay"
+	}
+	if _, err := exec.LookPath("ffplay"); err == nil {
+		return "ffplay"
+	}
+	return ""
+}
+
+func init() {
+	modules.Register(&SoundModule{})
+}