Fix race condition in TestSocketCleanup by protecting listener access with mutex

Fixes bd-160

The race was between Start() writing s.listener and Stop() reading it.
Now all listener access is protected by the server mutex:
- Start() stores listener under lock after creation
- Accept loop reads listener under RLock
- Stop() closes listener under lock

All RPC tests now pass with -race flag.

.gitignore

@@ -38,5 +38,10 @@ Thumbs.db
 .beads/daemon.pid
 .beads/bd.sock
 
+# .beads directory files (keep JSONL only)
+.beads/.gitignore
+.beads/db.sqlite
+.beads/bd.db
+
 # Keep JSONL exports (source of truth for git)
 !.beads/*.jsonl