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_unix.go

@@ -0,0 +1,18 @@
+//go:build unix
+
+package main
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// flockExclusive acquires an exclusive non-blocking lock on the file
+func flockExclusive(f *os.File) error {
+	err := unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if err == unix.EWOULDBLOCK {
+		return ErrDaemonLocked
+	}
+	return err
+}