chore: better structured Slack output for issue_pr_insights (#32503)
## Summary
- Truncate long titles (55 chars) with ellipsis to prevent Slack line
wrapping
- Add age indicator per item (e.g. `2h`, `1d`) for at-a-glance urgency
- Add 🔥 **Hot issues** section — surfaces issues with 3+ recent comments
(e.g. bugs reproducing for multiple people)
- Separate WIP PRs from ready-for-review PRs into distinct sections
- Human-readable date range header (`last 24h` or `Wed, Mar 4 → Thu, Mar
5`)
- When >20 total items, post a compact summary in the main message and
full details in a thread
- Hide empty sections entirely instead of showing "None"
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>