_bot_participated_threads was an in-memory set — lost on every restart.
After restart, the bot forgot which threads it was active in, requiring
fresh @mentions and potentially creating duplicate threads instead of
continuing existing conversations.
Changes:
- Persist thread IDs to ~/.hermes/discord_threads.json
- Load on adapter init, save on every new thread participation
- _track_thread() replaces direct .add() calls for atomic persist
- Cap at 500 tracked threads to prevent unbounded growth
- /thread slash command also tracks participation
- 7 new tests covering persistence, restart survival, corruption
recovery, cap enforcement
c8582fc4a2