[Fix] Fast sessions miss merge checkmarks from child PRs

[roomote-roomote]

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

What changed

Fast Slack sessions now receive the configured completion reaction on the thread root as soon as a delegated child task's pull request merge event is delivered. The reaction is independent of whether Fast posts a closeout or ignores a redundant status event; closed pull requests and unrelated platform events do not receive it.

Why this change was made

Regular Slack task threads already show a completion checkmark when their pull request merges, but Fast sessions did not inherit that terminal signal from child tasks. Tying the reaction to Fast's optional visible reply also allowed redundant merge events to complete without the checkmark.

Impact

Users can see at a glance that a Fast-delegated change has landed without opening the child task or pull request, even when Fast suppresses a redundant merge message. The existing closeout behavior remains unchanged, so this does not add duplicate terminal messages.

[roomote-community]

No code issues found. See task

• packages/sdk/src/server/lib/fast-agent-parent-event.ts:351 Merge reactions are skipped when the Fast agent ignores a redundant merge-status event.

_{Reviewed cf3f0c9}

[roomote-roomote]

Fixed and pushed in cf3f0c9. Merge reactions now run before the Fast agent decides whether to post or ignore the status event, with regression coverage for the ignored-event path.