When main goes red after a single pull landed, the forge opens a revert pull for it that names the failing test #6377

Part of #5994 (main can never silently go red). src/checks/red_main.ts pauses pull checks while main is red; nothing proposes the way back.

Done:

• [ ] When main's check goes red and the commit since the last green is one landed pull, the forge opens one pull that reverts it (using revertsPullId as src/core/pulls.ts already models). Its title names the pull, and its body names the failing test from the check summary.
• [ ] Never two revert pulls for one landing. No revert when the red spans more than one landing (the alarm covers that). The revert is proposed, never merged by itself.
• [ ] Tests: one landing then red opens one revert; a second tick opens none; two landings open none.

Files: src/checks/red_main.ts or a new src/checks/revert_red.ts, a test.

Risk: never touch the release workflow.

Before pushing: bun x tsc --noEmit and the tests you add or touch. Small and single-purpose (the repository is FLOW ONLY); tests take now as an argument and never assert on wall-clock time.