feat: Add Windows-compatible file locking for daemon

Replace Unix-only syscall.Flock with gofrs/flock library for
cross-platform file locking. This enables the daemon to run on
Windows in addition to Unix-like systems.

- Add github.com/gofrs/flock v0.13.0 dependency
- Replace syscall.Flock calls with flock.TryLock/Unlock
- Maintain same non-blocking exclusive lock semantics

(gt-5354h)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/daemon/daemon.go

@@ -13,6 +13,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/gofrs/flock"
 	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/boot"
 	"github.com/steveyegge/gastown/internal/config"
@@ -70,18 +71,19 @@ func (d *Daemon) Run() error {
 	// Acquire exclusive lock to prevent multiple daemons from running.
 	// This prevents the TOCTOU race condition where multiple concurrent starts
 	// can all pass the IsRunning() check before any writes the PID file.
+	// Uses gofrs/flock for cross-platform compatibility (Unix + Windows).
 	lockFile := filepath.Join(d.config.TownRoot, "daemon", "daemon.lock")
-	lock, err := os.OpenFile(lockFile, os.O_CREATE|os.O_RDWR, 0644)
-	if err != nil {
-		return fmt.Errorf("opening lock file: %w", err)
-	}
-	defer lock.Close()
+	fileLock := flock.New(lockFile)
 
 	// Try to acquire exclusive lock (non-blocking)
-	if err := syscall.Flock(int(lock.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+	locked, err := fileLock.TryLock()
+	if err != nil {
+		return fmt.Errorf("acquiring lock: %w", err)
+	}
+	if !locked {
 		return fmt.Errorf("daemon already running (lock held by another process)")
 	}
-	defer func() { _ = syscall.Flock(int(lock.Fd()), syscall.LOCK_UN) }()
+	defer fileLock.Unlock()
 
 	// Write PID file
 	if err := os.WriteFile(d.config.PidFile, []byte(strconv.Itoa(os.Getpid())), 0644); err != nil {