When lastAgentTimestamp was missing (new group, corrupted state, or
startup recovery), the empty-string fallback caused getMessagesSince to
return up to 200 messages — the entire group history. This sent a
massive prompt to the container agent instead of just recent messages.
Fix: recover the cursor from the last bot reply timestamp in the DB
(proof of what we already processed), and cap all prompt queries to a
configurable MAX_MESSAGES_PER_PROMPT (default 10). Covers all three
call sites: processGroupMessages, the piping path, and
recoverPendingMessages.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
