[Fix] Fast misses Slack follow-ups after peer mentions

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Telegram.

What changed

• Put the Slack peer-conversation admission bypass and quiet-participation hint behind a new per-user experiment that defaults off.
• Resolve the experiment from the owner of the bound Fast conversation, rather than the Slack user sending the current reply.
• Add a Slack peer conversations switch under Settings > Experimental, persisted in the conversation owner's personal preferences.
• Keep explicit Roomote mentions, linked-user authorization, unrelated workspace/channel/thread isolation, and legacy task-thread routing unchanged.

Why this change was made

The peer-conversation behavior changes which human messages reach Fast and can add inference cost. Keeping it opt-in preserves established Slack routing unless the owner of that Fast conversation deliberately enables the experiment.

Impact

With the experiment off or missing, peer mentions remain the existing admission cutoff and no peer-conversation hint is attached. With it on, eligible human discussion in an established Fast Slack thread reaches Fast, and current peer mentions add the quiet-participation hint while direct Roomote mentions continue normally.

The GitHub Test failure after the initial update was caused by two web client test fixtures that had not incorporated the new preference and Experimental-page child component. Those fixtures now cover the new false default and loaded/error rendering states. The full web client suite passes 300 files and 2,697 tests; focused Slack routing, preference persistence, durable follow-up delivery, conversation repository, and Fast service suites also pass. Full repository lint/type checking and pre-push validation pass. A final browser recapture was unavailable because the supervised Next.js development server retained stale Turbopack state after the large base merge; no final screenshot proof is claimed.

[roomote-community]

No new code issues found. See task

_{Reviewed 7dcc20d}

[roomote-roomote]

Pushed and verified fc419ff6: GitHub PR head and remote branch match that SHA. Removed priorMessages, latestDirectAddress, directlyAddressesRoomote, hasPeerDiscussion, the leading-name reset logic, and history-state tests/comments from fast-agent.ts / fast-agent-processing.test.ts.

The actual final diff retains the established-Fast-thread cutoff bypass in message-entry.ts and attaches the reminder solely from the current peer-mention message. Pre-existing history retrieval remains for ordinary context/attachments, not advisory reconstruction. Existing context delivery and small quiet-eligibility safeguards remain; no global prompt/schema/instruction-version redesign.

75 focused Slack tests and 103 SDK tests pass, plus affected typechecks and pre-push gates. The reminder is ordinary model context, not guaranteed persistent state; no live-model silence claim. Earlier unrelated DB-test schema failures remain disclosed in the refreshed PR body. PR not merged.