When a Slack user kicks off a PR review with `@open-swe review <pr-url>`,
the reviewer agent now posts a one-line summary back to the Slack thread
when it finishes — either "No issues found" or "found N potential
issue(s)" with a link to the GitHub review.
The reviewer agent has no Slack tools by design, so the summary is sent
host-side from `publish_review` after the GitHub review POST succeeds.
The Slack channel/thread_ts is persisted on reviewer thread metadata at
trigger time and read back on publish. Re-reviews triggered by push
events stay silent in Slack to avoid noise on the original thread.