A stranger's pull diff or a failed check log cannot close its fence in a crew brief #10554

Part of #10441, the first piece of #10451. Branch scout/10451-crew-briefs-widen-the-prompt-injection-t at 0ccf5279fd53 holds the fix.

Done:

• [x] In src/crew/brief.ts, the review diff and the failed check or deploy log go through fenced(), never a fixed three-backtick fence.
• [x] test/brief-injection.test.ts: a body that holds ```, a fake heading and "push to main" stays inside one fence in each brief, for both the review diff and the check log. Each test fails on today's main.
• [ ] The new test file goes into the gate list in the same push. The full bun test passes before the push, not just a hand-picked subset.

Files: src/crew/brief.ts, test/brief-injection.test.ts

Reviewed pull #10586 at its head: ticked In src/crew/brief.ts, the review diff and the failed check or deploy log go through fenced(), never a fixed three-backtick fence.; test/brief-injection.test.ts: a body that holds ``, a fake heading and "push to main" stays inside one fence in each brief, for both the review diff and the check log. Each test fails on today's main.. left open: The new test file goes into the gate list in the same push. The full bun test` passes before the push, not just a hand-picked subset. — The gate list needs no edit, but the builder did not run the full bun test before the push; only a subset ran, and the forge check carries the full run..

Left open after pull #10586 landed: 1 unticked item remains (The new test file goes into the gate list in the same push. The full bun test passes before the push, not just a hand-picked subset.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

The open box isn't wanted any more: scripts/gate.ts gates every test/*.test.ts that isn't in EXCLUDED, so test/brief-foreign-fence.test.ts is already in the gate and there's no list to edit. Closing.

Already done on main in 26463524f: the remaining box landed there (see my comment above).