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/notify/notify.go

@@ -0,0 +1,48 @@
+package notify
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type NotifyModule struct{}
+
+func (m *NotifyModule) Name() string {
+	return "notify"
+}
+
+func (m *NotifyModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Notify != nil && cfg.Notify.Enabled
+}
+
+func (m *NotifyModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	if stage != modules.StagePost {
+		return nil
+	}
+
+	title := "gosleep-timer"
+	body := fmt.Sprintf("Timer %s finished", ctx.Duration)
+	cmds := []string{fmt.Sprintf("notify-send %q %q", title, body)}
+
+	if ctx.Config.Notify.Sound != "" {
+		if _, err := exec.LookPath("paplay"); err == nil {
+			cmds = append(cmds, fmt.Sprintf("paplay %s", ctx.Config.Notify.Sound))
+		} else if _, err := exec.LookPath("aplay"); err == nil {
+			cmds = append(cmds, fmt.Sprintf("aplay %s", ctx.Config.Notify.Sound))
+		}
+	}
+
+	return cmds
+}
+
+func (m *NotifyModule) Validate() error {
+	_, err := exec.LookPath("notify-send")
+	return err
+}
+
+func init() {
+	modules.Register(&NotifyModule{})
+}