feat(runway): wire the git merger into the server

[behinddwalls]

Summary

Why?

The git merger exists but nothing constructs it — the server still builds the noop factory, so a deployed Runway acknowledges every merge as an instant success. This is the change that makes Runway actually merge.

It is deliberately last in the stack and deliberately opt-in: the switch is the presence of MERGE_CHECKOUT_PATH. Unset, the server keeps wiring noop, which is what local development, the compose stack, and the e2e suite depend on — none of them have a git checkout to hand the merger.

What?

newMergerFactory now reads the environment and returns either backend. With MERGE_CHECKOUT_PATH set it builds a git merger from the MERGE_* / GIT_* variables — remote, target branch, default strategy, committer identity, and the pinned git runtime — and logs the resolved configuration at startup. Without it, noop, with a log line saying so. A malformed configuration fails startup rather than degrading silently: parseStrategy rejects an unrecognized MERGE_DEFAULT_STRATEGY, and DEFAULT itself is rejected because it cannot be the value a DEFAULT step resolves to.

gitMergerFactory hands the same merger instance to every queue. The merger owns one checkout and serializes its own operations, so a second instance over the same directory would race; a deployment that lands multiple targets wires a per-queue map instead. This is also why the factory is built once at startup rather than per request.

Documents the new variables in the service README and adds the merger-extension overview to the Runway README.

Test Plan

✅ bazel build //service/runway/... — wiring compiles
✅ bazel test //runway/... — 6/6 pass

Not covered by automated tests: the git path only engages when MERGE_CHECKOUT_PATH points at a real checkout, so the env-to-Params mapping is exercised by the merger's own suite rather than through main. Watch the git merger configured startup log — with checkout, target, and default strategy — on the first deployment that sets the variable; its absence means the server silently fell back to noop.

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