fix(slack): dispatch bot-id-only mentions from the message handler

[yzxcj797]

Fixes the dispatch half of #630 (step 2 of the issue's fix sketch). Stacked on #632 (the recognition commit — mentionsBot/stripMention both-id forms come from there); merge that first. Together they close #630 end to end.

What still dropped after recognition

Even with mentionsBot recognizing the bot-id form, the message handler's channel path assumed app_mention would carry every mention: it returned before dispatch for top-level messages (if (!threadReply) return) and routed thread replies into the ambient stake-followed path (unprompted: true) when the mention wasn't the user-id form. Slack fires app_mention only for the bot-user-id form — the bot-id form had no dispatcher at all.

The dispatch

The message handler now dispatches the addressed turn itself, scoped to exactly the class app_mention won't fire for:

const botIdMentionOnly =
  Boolean(ids.ownBotId) &&
  text.includes(`<@${ids.ownBotId}>`) &&
  !(ids.botUserId && text.includes(`<@${ids.botUserId}>`));

• top-level bot-id-only mention → dispatched as an addressed turn (previously: mirrored handled=f, silently dropped);
• thread-reply bot-id-only mention → dispatched addressed — unprompted is deliberately omitted: the sender typed @qm; the ambient flag belongs to the stake-followed path alone (previously: misdispatched as ambient carrying the raw <@B…> token);
• both forms present → the user-id form routes through app_mention as before; the scoping makes the two paths disjoint, so no double dispatch (pinned by a test that emits both events for the same ts and asserts exactly one turn);
• neither form → unchanged ambient/stake-followed behavior.

The mirrored row reads handled for every dispatched mention — the incident's handled=f signature for a message the bot actually answered is gone.

Tests

• a bot-id-form top-level mention dispatches an addressed turn — turn dispatched, text stripped, no unprompted, mirror handled. Fails on the base (0 turns).
• a bot-id-form thread reply dispatches ADDRESSED, not ambient — unprompted undefined. Fails on the base (ambient dispatch).
• a user-id mention still routes through app_mention, not the message handler — both forms in one message, both events emitted, exactly one turn (disjointness).

Full identity suites (slack-message-gating + slack-mrkdwn + slack-identity + slack-index.integration): 120/120 on this stack; the two new dispatch tests fail on the recognition-only base (43/2). tsc --noEmit clean.

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