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)
+}

internal/lockfile/lock_wasm.go

@@ -4,7 +4,6 @@ package lockfile
 
 import (
 	"errors"
-	"fmt"
 	"os"
 )
 
@@ -13,5 +12,17 @@ var errDaemonLocked = errors.New("daemon lock already held by another process")
 func flockExclusive(f *os.File) error {
 	// WASM doesn't support file locking
 	// In a WASM environment, we're typically single-process anyway
-	return fmt.Errorf("file locking not supported in WASM")
+	return nil // No-op in WASM
+}
+
+// FlockExclusiveBlocking acquires an exclusive blocking lock on the file.
+// In WASM, this is a no-op since we're single-process.
+func FlockExclusiveBlocking(f *os.File) error {
+	return nil
+}
+
+// FlockUnlock releases a lock on the file.
+// In WASM, this is a no-op.
+func FlockUnlock(f *os.File) error {
+	return nil
 }

internal/lockfile/lock_windows.go

@@ -36,3 +36,34 @@ 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 {
+	// LOCKFILE_EXCLUSIVE_LOCK only (no FAIL_IMMEDIATELY = blocking)
+	const flags = windows.LOCKFILE_EXCLUSIVE_LOCK
+
+	ol := &windows.Overlapped{}
+
+	return windows.LockFileEx(
+		windows.Handle(f.Fd()),
+		flags,
+		0,
+		0xFFFFFFFF,
+		0xFFFFFFFF,
+		ol,
+	)
+}
+
+// FlockUnlock releases a lock on the file.
+func FlockUnlock(f *os.File) error {
+	ol := &windows.Overlapped{}
+
+	return windows.UnlockFileEx(
+		windows.Handle(f.Fd()),
+		0,
+		0xFFFFFFFF,
+		0xFFFFFFFF,
+		ol,
+	)
+}