land-stack: check a PR is not superseded before rebasing it

[EdbertChan]

Summary

Before land-stack rebases a PR or fixes its conflicts, it now runs a check that says whether the PR's work already landed.

If it did, the PR is closed instead of rebased, so newer work is not undone.

Review Claim

land-stack step 2 runs scripts/check_branch_not_superseded.py before any rebase or conflict fix: LIVE goes on, SUPERSEDED closes the PR, UNCHECKED is fixed and rerun.

Review Lane

behavior

Review Unit

product-skill

Safety Invariant

Only the skill's instructions and their test change. The step runs a check that only reads git state, and the step numbers stay 1 to 4.

Slice Rationale

This PR reads the check added by the PR below it, so it stacks on that one.

The check sits inside step 2 instead of a new step 3. A new step would renumber the later steps, and a router test fixture under engine/ pins those numbers, which would mix two review units in one PR.

Non-goals

• No new step number; the router fixture and tests/test_install.py are untouched.
• The land-bottom-up and check-each-merge steps are unchanged.
• Does not wire the check into a hook or CI.

Test Plan

Test Plan

• python3 -m unittest discover -s product/skills/land-stack/tests: Ran 21 tests ... OK
• python3 -m unittest discover -s engine/hooks/playbook-router/tests: Ran 29 tests ... OK
• python3 -m unittest tests.test_install.TestSkillSymlinks.test_playbook_router_linked_and_prompt_hook_wired_for_claude: OK
• python3 engine/skills/make-pr/scripts/preflight.py --base <the check's branch>: declare Review Unit: product-skill, ok preflight passed

Revert Plan

Revert Plan

• Safe to revert? Yes.
• Revert command: git revert <merge-sha>
• Post-revert steps: None. land-stack goes back to its current step 2.
• Data migration? No

🤖 Generated with Claude Code

https://claude.ai/code/session_01Gnua5cuAAF1ey1b7zRqJCC

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Note

Low Risk
Only land-stack skill instructions and documentation tests change; the new step invokes an existing read-only git gate and does not alter merge/queue behavior.

Overview
land-stack step 2 now requires running scripts/check_branch_not_superseded.py on each PR after verify_stack.py and before any rebase or conflict resolution, still without adding a new numbered step.

The skill documents git fetch, the path via the skill symlink into catstack, and how to interpret exits: LIVE (0) continues, SUPERSEDED (3) means close the PR with a comment instead of rebasing, UNCHECKED (2+) must be fixed and rerun and must not be treated as LIVE.

A new unittest module locks that prose to the repo: gate script location, ordering between verify and land-bottom-up, EXIT_CODES parity with the gate module, close-not-rebase for superseded, and the fail-closed rule for unchecked.

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