feat(runway): git merger PROMOTE

[behinddwalls]

Summary

Why?

PROMOTE is the last strategy in the wire contract without an apply path. It is also the one that does not fit the shared machinery: the transforming strategies build new commits locally and push HEAD:target, while PROMOTE advances the target to a commit that already exists, unchanged.

What?

Adds the promote path, dispatched directly from process rather than through applyTransforming.

Fast-forward only. After resetting to the remote tip, promote classifies the named commit three ways. Already the tip, or contained in it — idempotent success, no push. A strict descendant of the tip — a genuine fast-forward, pushed as <sha>:refs/heads/<target>. Anything else has diverged and is a terminal ErrConflict; PROMOTE never creates a commit to reconcile the two.

Exclusivity. resolveAndValidate rejects a PROMOTE that is not the entire request — one step, one change, one URI — as ErrInvalidRequest. Two reasons, both structural: a pre-existing commit cannot descend from commits an earlier transforming step just produced, and the push targets an exact SHA rather than the locally-built HEAD, so there is nothing for a preceding step to contribute.

Contention. The same bounded retry as the transforming path, but the loop re-runs the classification rather than the apply: if the push is rejected the tip may have moved, and the commit that was a fast-forward a moment ago may now be contained (success) or divergent (conflict).

The push is a single atomic ref update, so PROMOTE needs no separate atomicity argument — there is no window in which the remote holds a partial result. A dry-run check performs the identical classification and returns without pushing, reporting no output.

With this the merger implements every strategy in the contract; isConcreteStrategy now admits all four.

Test Plan

✅ bazel test //runway/... — 6/6 pass (git suite 32.1s)

New cases: fast-forward promote, promote of a commit already contained in the tip, divergent promote rejected as a conflict, both dry-run classifications, and the two composition rules (PROMOTE with a second step, PROMOTE with a second URI) rejected as invalid requests.

Stack

1. chore(submitqueue): remove dead pusher extension #457
2. feat(runway): classify invalid merge requests as terminal #458
3. feat(runway): dlq reconciler for merge topics #459
4. feat(runway): git-backed merger with REBASE #460
5. feat(runway): git merger SQUASH_REBASE and MERGE #461
6. @ feat(runway): git merger PROMOTE #462
7. feat(runway): wire the git merger into the server #463