Fail pull requests that add agent job folders or hand-off notes #712

Subfleet build jobs keep their state in a .job/ folder at the root of their worktree: HANDOFF.md, PR_BODY.md, env, a HEAD file, git bundles and sometimes a whole repo.git. On 23 October 2026 these files reached pull requests here: .job/HANDOFF.md in #704, and .job/HANDOFF.md plus .job/PR_BODY.md in #699. A UK certification prep branch also tracked its whole .job/ toolkit. They were removed by hand (#704 at 463c3a9, #699 at c669bbd).

Nothing in CI would have stopped them reaching main. Also, pr_code_changes.yaml has a paths: filter, so a pull request that adds only .job/ files or root notes triggers no workflow at all.

policyengine-uk already has a guard for root-level agent notes (its pr_code_changes.yaml, the "No agent working notes at the repository root" step).

Proposed fix:

• A small workflow that runs on every pull request, with no path filter, and fails when the merged tree contains:
  • .job/, .job-git/ or .diag/ at any depth;
  • root-level HANDOFF.md, handoffs.jsonl, PR_BODY.md, PR-BODY.md, REPORT.md or PROGRESS.md, with any capitalisation.
• .gitignore entries for the three folders.