Get buildable summary by revision

[albertywu]

Add arc buildable command to fetch Harbormaster buildable summaries for Differential revisions.

---

 

[cursor[bot]]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch. _{Learn more about Cursor Agents}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}