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/workspace/workspace.go

@@ -0,0 +1,56 @@
+package workspace
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type WorkspaceModule struct{}
+
+func (m *WorkspaceModule) Name() string {
+	return "workspace"
+}
+
+func (m *WorkspaceModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Workspace != nil && cfg.Workspace.Enabled
+}
+
+func (m *WorkspaceModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	if stage != modules.StagePre {
+		return nil
+	}
+
+	backend := ctx.Config.Workspace.Backend
+	if backend == "" || backend == "auto" {
+		backend = config.DetectDesktop()
+	}
+
+	n := ctx.Config.Workspace.Workspace
+	switch backend {
+	case "niri":
+		return []string{fmt.Sprintf("niri msg action focus-workspace %d", n)}
+	case "hyprland":
+		return []string{fmt.Sprintf("hyprctl dispatch workspace %d", n)}
+	case "kde":
+		return []string{fmt.Sprintf("qdbus org.kde.KWin /KWin setCurrentDesktop %d", n)}
+	default:
+		return nil
+	}
+}
+
+func (m *WorkspaceModule) Validate() error {
+	// Check common workspace binaries
+	for _, bin := range []string{"niri", "hyprctl", "qdbus"} {
+		if _, err := exec.LookPath(bin); err == nil {
+			return nil
+		}
+	}
+	return fmt.Errorf("workspace: no supported backend binary found (niri, hyprctl, qdbus)")
+}
+
+func init() {
+	modules.Register(&WorkspaceModule{})
+}