fix(merge): push restacked branches after each stack merge step

[rohoswagger]

What changed

After each successful ez merge (single PR or each step of ez merge --stack), any branch that was restacked locally is git fetch + git push --force-with-lease’d so GitHub’s PR head matches the rebased commits before the next merge runs. That fixes stack merges failing on the next PR with merge conflicts until you manually ran ez restack and force‑pushed.

Merge receipts JSON now include restacked and pushed_branches per step.

ez merge --help doc line notes the push behavior.

Example: ez merge --stack --yes

Stack

Branch	Parent	PR
feat/auth	main	#180
feat/api	feat/auth	#181
feat/ui	feat/api	#182

Run from tip (or any branch in the line):

ez merge --stack --yes --method squash

ez merges bottom → top: [feat/auth, feat/api, feat/ui].

Round 1 (#180)

1. Squash-merge PR #180 on GitHub.
2. Reparent dependents onto main, try gh pr edit --base main for child PRs.
3. Cleanup local/remote merged branch, git fetch.
4. Restack branches whose parent_head is stale (e.g. feat/api, feat/ui onto updated main).
5. Push each branch that was restacked (--force-with-lease).

Round 2 (#181) — same pattern; push restacked dependents (e.g. feat/ui) so PR #182 is mergeable.

Round 3 (#182) — merge last PR; restack/push if anything still moved.

Single PR

ez merge --yes --method squash

Merges only the stack bottom PR for your current line; the global restack + push loop still runs afterward so dependents stay aligned with trunk.

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--method is GitHub’s merge_method: squash (default), merge, or rebase.