fix(chat): three concurrency holes found reviewing my own patch (v4.6.77)

1. chat.py renamed the temp file AFTER releasing the flock, so two writers could each
   read the pre-replace snapshot and the later rename silently dropped the earlier
   message. os.replace now happens inside the lock.
2. The agent cursor was global. After hermes-reset it pointed past the end of the new
   empty thread, so every read returned nothing — the agent would think it had been
   ignored. Key is now per thread and reset clears every chat cursor.
3. chat-relay.sh had no overlap guard: a slow agent run (up to 5 min) could collide with
   the next tick and answer the same thread twice. flock -n on /tmp/chat-relay.lock.

Verified: stale cursor 'webx11_chat_agent_cursor_oldthread00000' set to 9999999999999,
hermes-reset -> zero cursor keys remain, read returns {messages:[], note:'no new messages...'}.

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