[Fix] Fast mode replies to ambient participant conversation

[roomote-roomote]

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

What changed

• Allows Fast mode to end ambient multi-participant thread turns silently instead of forcing a reply.
• Preserves responses for explicit Roomote mentions, slash commands, direct messages, and single-participant conversations.
• Removes eager processing reactions and suppresses misleading reply-target context only for ambient shared-thread turns, while directed turns retain the latest Roomote reply.
• Applies the same directedness behavior to Slack and Discord Fast conversations.

Why this change was made

Fast mode treated every routed human message as requiring a visible response, which made it interrupt conversations between other thread participants. Standard task threads already distinguish messages directed at Roomote from passive thread activity.

Impact

Roomote can remain silent when people are talking to each other in an established Fast thread, while continuing to answer messages that are actually addressed to it. New participants can still join the Fast conversation naturally without losing explicit mention handling.

[roomote-community]

1 issue outstanding. See task

• Preserve <replying_to> context for directed Fast turns in shared threads.
• Preserve responses to native Discord replies to Roomote in shared Fast threads.

_{Reviewed 855eb8a}

[roomote-community]

A native Discord reply to a Roomote message has a message_reference but need not contain an @mention. In a shared Fast thread this passes directedAtRoomote: false, so ignore_message becomes valid and the model can silently discard a reply that was directly addressed to Roomote. Classify replies whose referenced message is Roomote-authored as directed as well (and add a regression test).