fix(daemon): add cross-process locking to registry (bd-5bj)

The global daemon registry (~/.beads/registry.json) could be corrupted
when multiple daemons from different workspaces wrote simultaneously.

Changes:
- Add file locking (flock) for cross-process synchronization
- Use atomic writes (temp file + rename) to prevent partial writes
- Keep entire read-modify-write cycle under single lock
- Add FlockExclusiveBlocking and FlockUnlock to lockfile package

This prevents race conditions that caused JSON corruption like `]]`.

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

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

internal/lockfile/lock_unix.go

@@ -19,3 +19,14 @@ func flockExclusive(f *os.File) error {
 	}
 	return err
 }
+
+// FlockExclusiveBlocking acquires an exclusive blocking lock on the file.
+// This will wait until the lock is available.
+func FlockExclusiveBlocking(f *os.File) error {
+	return unix.Flock(int(f.Fd()), unix.LOCK_EX)
+}
+
+// FlockUnlock releases a lock on the file.
+func FlockUnlock(f *os.File) error {
+	return unix.Flock(int(f.Fd()), unix.LOCK_UN)
+}