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/custom/custom.go

@@ -0,0 +1,34 @@
+package custom
+
+import (
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type CustomModule struct{}
+
+func (m *CustomModule) Name() string {
+	return "custom"
+}
+
+func (m *CustomModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Custom != nil && cfg.Custom.Enabled
+}
+
+func (m *CustomModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	switch stage {
+	case modules.StagePre:
+		return ctx.Config.Custom.Pre
+	case modules.StagePost:
+		return ctx.Config.Custom.Post
+	}
+	return nil
+}
+
+func (m *CustomModule) Validate() error {
+	return nil
+}
+
+func init() {
+	modules.Register(&CustomModule{})
+}