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/registry.go

@@ -0,0 +1,53 @@
+package modules
+
+import "github.com/forust/gosleep-timer/config"
+
+var registry []Module
+
+func Register(m Module) {
+	registry = append(registry, m)
+}
+
+func GetAll() []Module {
+	return registry
+}
+
+func GetEnabled(cfg *config.Modules) []Module {
+	var enabled []Module
+	for _, m := range registry {
+		if m.Enabled(cfg) {
+			enabled = append(enabled, m)
+		}
+	}
+	return enabled
+}
+
+func CollectPreCmds(ctx *ModuleContext) []string {
+	var cmds []string
+	for _, m := range GetEnabled(ctx.Config) {
+		cmds = append(cmds, m.BuildCmds(ctx, StagePre)...)
+	}
+	return cmds
+}
+
+func CollectPostCmds(ctx *ModuleContext) []string {
+	var cmds []string
+	for _, m := range GetEnabled(ctx.Config) {
+		cmds = append(cmds, m.BuildCmds(ctx, StagePost)...)
+	}
+	return cmds
+}
+
+func ValidateAll() []error {
+	var errs []error
+	for _, m := range registry {
+		if err := m.Validate(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return errs
+}
+
+func Init() {
+	// registry starts empty; modules register themselves via init() or Register()
+}