Fix bd-54: Prevent multiple daemon instances via file locking

- Implemented daemon.lock using flock (Unix) and LockFileEx (Windows)
- Lock acquired before PID file, held for daemon lifetime
- Eliminates race conditions in concurrent daemon starts
- Backward compatible: falls back to PID check for old daemons
- Updated isDaemonRunning() to check lock availability
- All tests pass including new lock and backward compatibility tests

Amp-Thread-ID: https://ampcode.com/threads/T-0e2627f4-03f9-4024-bb4b-21d23d296300
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/daemon_lock_windows.go

@@ -0,0 +1,35 @@
+//go:build windows
+
+package main
+
+import (
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// flockExclusive acquires an exclusive non-blocking lock on the file using LockFileEx
+func flockExclusive(f *os.File) error {
+	// LOCKFILE_EXCLUSIVE_LOCK (2) | LOCKFILE_FAIL_IMMEDIATELY (1) = 3
+	const flags = windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY
+
+	// Create overlapped structure for the entire file
+	ol := &windows.Overlapped{}
+
+	// Lock entire file (0xFFFFFFFF, 0xFFFFFFFF = maximum range)
+	err := windows.LockFileEx(
+		windows.Handle(f.Fd()),
+		flags,
+		0,             // reserved
+		0xFFFFFFFF,    // number of bytes to lock (low)
+		0xFFFFFFFF,    // number of bytes to lock (high)
+		ol,
+	)
+
+	if err == windows.ERROR_LOCK_VIOLATION || err == syscall.EWOULDBLOCK {
+		return ErrDaemonLocked
+	}
+
+	return err
+}