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>

README.md

@@ -754,6 +754,8 @@ Issue is "ready" if status is `open` and it has no open `blocks` dependencies.
 beads/
 ├── cmd/bd/              # CLI entry point
 │   ├── main.go          # Core commands (create, list, show, update, close)
+│   ├── daemon.go        # Background daemon for auto-sync
+│   ├── daemon_lock.go   # Daemon exclusivity enforcement
 │   ├── init.go          # Project initialization
 │   ├── quickstart.go    # Interactive guide
 │   └── ...
@@ -764,6 +766,17 @@ beads/
 └── EXTENDING.md         # Database extension guide
 ```
 
+### Daemon Locking
+
+bd ensures only one daemon runs per repository using file locking:
+
+- **Lock file**: `.beads/daemon.lock` holds an exclusive file lock (via `flock` on Unix, `LockFileEx` on Windows)
+- **Process-level exclusivity**: The lock is held by the daemon process for its entire lifetime
+- **Automatic cleanup**: Lock is automatically released when daemon exits (via file descriptor close)
+- **Race condition prevention**: Multiple simultaneous daemon starts are safely serialized by the lock
+
+The lock file is the single source of truth for daemon status. Commands like `bd daemon --status` check lock availability rather than PID file contents, eliminating race conditions where multiple daemons could start simultaneously.
+
 ## Extending bd
 
 Applications can extend bd's SQLite database with their own tables. See [EXTENDING.md](EXTENDING.md) for the full guide.