In multi-agent shared Matrix rooms, multiple bots all participating in the
same thread could trigger infinite reply loops — each bot's reply re-engaged
the others because they were all in the bot-thread set. Discord has a
`thread_require_mention` opt-in for this; Matrix didn't.
Add `_parse_thread_require_mention(config)` (mirrors Discord's pattern).
In `_resolve_message_context`, when enabled and the message is in a
bot-participated thread (not a free-response room), require @mention
before processing.
Salvage of @justemu's 2-commit stack (#27996). Fixes#27995.
276e6cc52d