Register go/chalkline + announce internally

- [ ] Register go/chalkline → https://github.com/block/chalkline
- [ ] One-liner in #opensource (the OSPO issue already introduced it: squareup/ospo#1084)
- [ ] Builder Labs channel note: the configuration front door exists; 30-minute setup; field reports → setup-experience issues
- [ ] Mark the next-step complete in the Clay/David 1:1 doc and link the repo

🤖 Seeded by @daveh-beep's agent while standing up the repo.

Start with the checklist in issue #7 and review the linked chalkline repository and squareup/ospo#1084 for the intended wording and context. Done means completing the repository registration, both internal announcements, and the Clay/David 1:1 documentation update with the repository link.

Written by the indexing model from the issue text.