Ship gate: merge-check --ship fails while a ship-blocker is open #348

Part of epic #337 (linked as parent). The owner approved the epic and all its batches on 2026-10-10, including this merge-check change under clause 2; this child is batch 8 and inherits that approval.

User story

As the owner, I want a release to wait for every open ship-blocker and a green full run on main, so that nothing must-fix ships.

Acceptance criteria

• merge-check --ship exits 1 and names every open issue labelled ship-blocker.
• merge-check --ship fails and names each ship-blocker that also carries deferred or parked.
• merge-check --ship fails when the repo has no ship-blocker label.
• In an opted-in repo, merge-check --ship fails without a green devbox/verify-full on the default-branch head.
• In an opted-in repo, merge-check --ship fails without a green push CI run on the default-branch head.
• merge-check --ship exits 0 only when every result passes or reads NA.
• merge-check --ship refuses a PR number, --epic, --merge and --close.
• A bug issue states ship-blocker: yes or no against the three criteria in the epic's Context.

What I'd notice: a new merge-check --ship that answers SHIPPABLE or BLOCKED and names the blockers.

Tasks

• --ship in merge-check (scratch copy, then install on both machines).
• Unit tests with gh_json and gh_text mocked, per the contract's test list.
• CONTRACT.md: --ship and the ship-blocker criteria.
• The planner's issue-writing rule: every bug states ship-blocker: yes or no.

Contract

The planner's contract is the comment on #337, section 5 and child E. Clause 2: yes, approved by the owner on 2026-10-10. This child has no relay PR; it closes with a comment naming the installed md5s.

Ownership (outside the repo): ~/.claude/merge-check/merge-check (dev), ~/.claude/merge-check/test_merge_check.py (test writer), ~/.claude/merge-check/CONTRACT.md (coordinator), ~/.claude/agents/planner.md (issue-writing rule).

Budget: 4 files, about 350 lines. Depends on child D's merge-check install (#344).

Waits: none; tests are pure functions over mocked gh.

Decisions made for the owner

• See the contract comment on Epic: PRs run only the e2e tests their change affects, with a full run at epic close #337.