feat(stovepipe): Promote green commits to a ref

[prathshenoy]

Summary

What:

• When a validation establishes a commit as green, the record stage now asks source control to advance the queue's promotion ref to that commit, so downstream systems can fetch the latest known-good commit by branch name. Which ref a queue promotes to, and whether it promotes at all, is source-control configuration resolved from the queue name.
• Promotion happens only while the request still holds the queue's last-green bookmark, and a commit that a rewritten history dropped from the branch is counted and skipped rather than retried.

Why:

• Consumers that only need "where is this branch last green" had to query Stovepipe and understand requests, commit locators and greenness degrees; resolving a branch name lets a consumer get the same answer with a plain fetch.
• Reusing the bookmark's existing ordering guard keeps the ref forward-only without new state, and skipping commits that left the branch avoids dead-lettering work whose greenness is already durable.

Test Plan

• Add unit tests.

Revert Plan

• Revert this PR. The only residue is the ref sitting at whatever commit it last landed on.

Issues

• CODEM-435

[mnoah1]

Looks good - before merging, go ahead and implement the Promote endpoint on our internal source control extension, so the version bump job succeeds.