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/lock/lock.go

@@ -0,0 +1,36 @@
+package lock
+
+import (
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type LockModule struct{}
+
+func (m *LockModule) Name() string {
+	return "lock"
+}
+
+func (m *LockModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Lock != nil && cfg.Lock.Enabled
+}
+
+func (m *LockModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	if stage != modules.StagePost {
+		return nil
+	}
+	cmd := ctx.Config.Lock.Command
+	if cmd == "" {
+		cmd = "loginctl lock-session"
+	}
+	return []string{cmd}
+}
+
+func (m *LockModule) Validate() error {
+	// No strict validation — custom commands may be set
+	return nil
+}
+
+func init() {
+	modules.Register(&LockModule{})
+}