refactor(pusher): push ordered batches, return per-batch outcomes

[behinddwalls]

Summary

Change Pusher.Push to take ordered []entity.Batch instead of controller-pre-resolved []entity.Change, per the extension contract. The git pusher and the fake gain an injected changeset.Resolver and resolve each batch's changes themselves; the merge controller drops its private collectChanges walk and passes the single batch (the list designs for a future merge-train).

This is the one extension whose output shape also changes: Result now groups outcomes per batch — Result{Batches []BatchOutcome}, where BatchOutcome{BatchID, Outcomes []ChangeOutcome} — so each landed batch stays correlatable, the way conflict.Conflict carries its BatchID. ChangeOutcome (per-change commit detail) is unchanged. No per-batch status: push atomicity stays all-or-nothing across the whole call.

Test Plan

Issues

Stack

1. docs(rfc): extension contract — identity in, resolve internally #214
2. feat(changeset): shared batch→changes resolver in core #216
3. refactor(mergechecker): accept entity.Request, resolve change internally #217
4. refactor(changeprovider): accept entity.Request, resolve change internally #218
5. refactor(scorer): score entity.Batch, resolve changes internally #219
6. refactor(buildrunner): trigger on batches, resolve changes internally #221
7. @ refactor(pusher): push ordered batches, return per-batch outcomes #222
8. feat(conflict): target-overlap analyzer using changeset resolver #223