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/mute/mute.go

@@ -0,0 +1,53 @@
+package mute
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type MuteModule struct {
+	tool string
+}
+
+func (m *MuteModule) Name() string {
+	return "mute"
+}
+
+func (m *MuteModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Mute != nil && cfg.Mute.Enabled
+}
+
+func (m *MuteModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	switch stage {
+	case modules.StagePre:
+		if m.tool == "wpctl" {
+			return []string{"wpctl set-mute @DEFAULT_AUDIO_SINK@ 1"}
+		}
+		return []string{"pactl set-sink-mute @DEFAULT_SINK@ 1"}
+	case modules.StagePost:
+		if m.tool == "wpctl" {
+			return []string{"wpctl set-mute @DEFAULT_AUDIO_SINK@ 0"}
+		}
+		return []string{"pactl set-sink-mute @DEFAULT_SINK@ 0"}
+	}
+	return nil
+}
+
+func (m *MuteModule) Validate() error {
+	if _, err := exec.LookPath("pactl"); err == nil {
+		m.tool = "pactl"
+		return nil
+	}
+	if _, err := exec.LookPath("wpctl"); err == nil {
+		m.tool = "wpctl"
+		return nil
+	}
+	return fmt.Errorf("mute: neither pactl nor wpctl found in PATH")
+}
+
+func init() {
+	modules.Register(&MuteModule{})
+}