docs: document re-running the WordPress.com Tests CI check via context-a8c

[kraftbj]

Proposed changes

• Adds a "Re-running CI Checks" subsection to AGENTS.md so coding agents are aware of the newly announced ability to re-run the WordPress.com Tests check (the TeamCity JetpackPreFlightChecks_BasicChecks build) — which can't be restarted via the GitHub UI or the gh CLI. Agents now know to use /context-a8c:rerun-jetpack-preflight <PR-number> for that one check and keep using the usual gh commands for everything else.

Related product discussion/links

• pdWQjU-1Ch-p2

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Read the new "Re-running CI Checks" subsection under "Pull Requests" in AGENTS.md and confirm it accurately describes the command.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!