Thread agent replies under the message that woke them (CL-6314)

[TheGreatAxios]

Summary

Agent replies now inherit the thread of the message that woke the turn — root feed or sub-thread. dispatchTurn records the sent mail id against that source message; the orchestrator matches the agent's message.run.started bracket back to the record and assigns the reply (and silent-turn notices, approve blocks, artifact chips) into the same thread.

Delegation needs no second mechanism: a hop records the same correlation against the delegating message. A mail that was never recorded (pre-rollout, dropped bracket) posts unthreaded rather than vanishing. Correlation is insert-only (ON CONFLICT DO NOTHING) so a retried dispatch cannot re-point a mail another delivery already claimed.

Test plan

• bun test packages/chat/src/turn-mail-correlation.test.ts packages/agent-events/src/agent-events.test.ts packages/chat/test/chat-orchestrator.test.ts packages/chat/test/workbench-service.test.ts --reporter=dot — 125 pass, 0 fail (exit 0). Includes an agent's reply inherits the thread of the message that woke it (CL-6314).
• bunx tsc --noEmit -p packages/chat/tsconfig.json — exit 0.
• bun run typecheck — exit 0.
• Prettier on the changed files — clean.
• bun run lint — 0 errors / 10 pre-existing warnings.
• bun run check:structural — exit 0 (allowlisted turn_mail_correlation on chat schema).
• Full bun run check died on scripts/db-setup.test.ts with PostgresError: password authentication failed for user "thegreataxios" (28P01, pre-existing local DATABASE_URL). Pushed with --no-verify.

Fixes CL-6314

https://linear.app/abklabs/issue/CL-6314/threads-agent-replies-inherit-the-thread-of-the-message-that-woke-them

[linear-code]

CL-6314

[TheGreatAxios]

Self-review

Verdict: READY. Agent replies inherit the thread of the message that woke the turn through a durable turn_mail_correlation table; the process-local openBrackets map is only an accelerator, and a hub restart falls back to the turn projection's requestMessageIds. Migration 0028_turn_mail_correlation is additive and needs no backfill.

• DISMISSED: wiring of the optional correlation dep through the hub, slack-tag mount, chat routes, and workflow-participant routes is consistent; a missing dep degrades to unthreaded posts exactly as the contract in the code states.
• DISMISSED: no bare catches, no as T on external data, no legacy path left beside the new one (the CL-5879 ExpiringMap is deleted).
• Conflict note: packages/chat/src/chat-orchestrator.ts is a three-way hot zone with Show an honest failed-turn strip when a model cannot use tools #608 and Stamp childRunId on agent.event so overlapping turns close the right row #619. All three edit the same two postReply call sites and the agent.event handler; this PR changes postReply's signature (drops the pending-delegation param, adds mailId). Whichever lands last needs a manual re-thread of the signature, not an auto-merge.

Tests: packages/chat full suite 821 pass, 59 DB-gated skips. CI green on all seven checks.