chore(submitqueue): remove dead pusher extension

[behinddwalls]

Summary

Why?

The pusher extension is dead code. Merge execution moved to Runway, where the pluggable contract is runway/extension/merger — a Runway-owned interface that takes a MergeRequest off the queue rather than SubmitQueue entity.Batch values. Nothing in the tree imports submitqueue/extension/pusher any more: the only remaining references were the package's own files and the mocks target in the Makefile.

Removing it now keeps a second, divergent merge contract from drifting alongside the live one.

What?

Deletes the whole submitqueue/extension/pusher tree — the Pusher/Factory contract, the git-backed implementation, the fake, and the generated mock.

Drops ./submitqueue/extension/pusher/... from the go generate invocation in the mocks Makefile target. Left in place it would point make mocks at a path that no longer exists.

Drops the stale pusher mention from the changeset package docs, which listed it as an example of a decision/action extension resolving content through a Resolver.

Test Plan

✅ make gazelle — no BUILD churn beyond the deleted packages
✅ make fmt
✅ make test — 77/77 pass
✅ grep -rn "extension/pusher" over the tree returns nothing

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