#20567 docs: gh-merge-green is optional on feat-cmux-next (no bead)

Lawrence and Leo: gh-merge-green is optional on feat-cmux-next, and nothing should tell agents to use it. A plain gh pr merge --squash --match-head-commit <sha> under the branch's rules is fine. The helper's fast gate is in #20566 (main, where the helper lives).

• docs/ci/cmux-next-batch.md: by hand, the owner lands a validated PR with a plain merge or the optional helper. Only the queue's own --land uses gh-merge-green.
• scripts/ci/next_batch.py: the green receipt now says "land it yourself on this receipt (gh pr merge --squash --match-head-commit; gh-merge-green is optional)".

Verification: python3 tests/test_next_batch.py passes (it asserts "land it yourself" in the receipt).

Changelog

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Summary by cubic

Clarifies that gh-merge-green is optional on feat-cmux-next and that nothing should tell agents to use it. A plain gh pr merge --squash --match-head-commit <sha> is fine under the branch's rules.

• Updates docs/ci/cmux-next-batch.md so only the queue's own --land uses gh-merge-green; owners land validated PRs with a plain merge.
• Changes the green receipt in scripts/ci/next_batch.py to say "land it yourself on this receipt" and note that gh-merge-green is optional.