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

engine/executor.go

@@ -0,0 +1,42 @@
+package engine
+
+import (
+	"context"
+	"os/exec"
+	"time"
+)
+
+type ExecResult struct {
+	Cmd     string
+	Output  string
+	Err     error
+	Elapsed time.Duration
+}
+
+func Execute(ctx context.Context, cmd string, timeout time.Duration) ExecResult {
+	start := time.Now()
+
+	var cancel context.CancelFunc
+	if timeout > 0 {
+		ctx, cancel = context.WithTimeout(ctx, timeout)
+		defer cancel()
+	}
+
+	out, err := exec.CommandContext(ctx, "bash", "-c", cmd).CombinedOutput()
+	elapsed := time.Since(start)
+
+	return ExecResult{
+		Cmd:     cmd,
+		Output:  string(out),
+		Err:     err,
+		Elapsed: elapsed,
+	}
+}
+
+func ExecuteAll(ctx context.Context, cmds []string, timeout time.Duration) []ExecResult {
+	results := make([]ExecResult, len(cmds))
+	for i, cmd := range cmds {
+		results[i] = Execute(ctx, cmd, timeout)
+	}
+	return results
+}