vet-foreign-pr-mergeabilitylisted

# Vet a Foreign PR's Mergeability ## Overview A PR you didn't write needs a real merge-test, not just a green-checks glance. Test the merge in an isolated worktree, run the build gate on the result, review the diff's content, confirm it stays in its lane, then report — and re-verify live state first, because an "open" PR can already be merged by a concurrent session (reverify claimed state against live source). ## When to use - "investigate this open PR — can it be merged?" / "is PR #N safe to merge?" - merging a sister session's PR in a multi-lane repo - any PR whose diff you haven't authored and must vouch for Distinct from `multi-pr-sweep-pipeline` (ships YOUR N near-identical PRs) and pr check rollup vs required gate (explains red rollups). ## Steps 1. **Re-verify it's still open & not already in master:** GitHub MCP `pull_request_read` + `git merge-base --is-ancestor origin/<branch> origin/master`. 2. **Merge-test in a throwaway worktree:** `git worktree add /tmp/vet-<pr> origin/master`; in it `git merge --no-edit origin/<branch>` — note conflicts vs "Already up to date". For a read-only conflict preview with no worktree at all, `git merge-tree --write-tree --name-only origin/master origin/<branch>` enumerates the conflicting files without touching any tree; still use the full worktree when you also need to run the gate. 3. **Run the repo's gate** on the merge result (e.g. `mkdocs build --strict`, the test suite). 4. **Content-review in parallel:** dispatch a suba