Make release Slack QA notification compact and @-mention flagged-PR authors

[nollymar]

Background

The release-pipeline Slack QA notification (#35762) launched with two real-world problems uncovered on the first live run:

1. The detailed PR list got clipped — Slack auto-collapses messages over ~4000 chars, so the per-PR list ended up hidden behind a Show more link. Reviewers never saw which PRs needed attention.
2. Author mentions didn't notify — plain @username is decorative text; without a Slack member ID wrapped in <@U...>, the person whose action was required got no notification.

Scope

• Replace the detailed PR list in the Slack message with a compact summary: the four bucket counts (linked to the workflow run summary) plus a single cc line that @-mentions the authors of flagged PRs.
• Resolve GitHub usernames to Slack member IDs via .github/data/slack-mappings.json (the same file utility_slack-channel-resolver.yml uses). Fall back to plain @login when the user isn't in the mapping.
• Move the full detailed report (PR-by-PR tables) to the workflow run summary via $GITHUB_STEP_SUMMARY, so reviewers can drill in from the count links.
• Add stable per-bucket anchors so each count deep-links directly to its bucket section in the summary.

Out of scope

• Maintaining the slack-mappings.json over time — that's a separate ongoing task.
• Cross-repo issue label visibility (PRs that close dotCMS/private-issues#N still surface in the "outside dotCMS core" bucket because the labels aren't readable from here).

Related

• Initial implementation: PR feat(release): warn in Slack about PRs in a release that lack QA verdict #35762 (merged)
• This work: PR feat(release): compact Slack QA section + @-mention flagged-PR authors #35815

[github-actions]

PRs linked to this issue

• feat(release): compact Slack QA section + @-mention flagged-PR authors by @nollymar