One test seeds a clog (51 opened against 15 landed, six pulls on one file) and proves the governor and the hot-file hold prevent it #6523

Part of #5930, done-when: "a seeded clog is prevented by the governor and the WIP limit". The pieces are on main and tested alone; nothing proves them together.

Done:

• [ ] test/flow-clog-prevent.test.ts seeds vitals of 51 opened and 15 landed in the last hour, and remembers six open pulls touching one file through rememberPullFiles.
• [ ] It asserts governedSlots lowers the slots below the ceiling with a reason naming the numbers, and heldBy holds a fresh issue naming that file as hot_file, while an issue naming another file is not held.
• [ ] No wall clock; under 1 s.

Files: test/flow-clog-prevent.test.ts.

Before pushing: bun x tsc --noEmit and the new test.