Every test file is in the gate or excluded by name with an owning issue; the gate's list leaves forge.json (first half of #261) #492

Every test file is accounted for: it is either in the push gate or on a named exclusion list with a reason and an owning issue. This step changes nothing about what the gate runs.

This is the first, smaller half of #261. Pull #410 tried to widen the gate and guard it in one step, and it went red on files outside the known table. Here the list moves out of forge.json and gets its guard, and the set of files the gate runs stays exactly the same, so nothing new can go red.

Done looks like

• [ ] scripts/gate.ts exports GATE, the exact 30-odd files forge.json's check.run names today, and EXCLUDED, every other test/*.test.ts as { file, reason, issue }. Most entries read reason: "not yet admitted", issue: 261. The known-red files carry their owners from #261's table (#290, #291, #292, #293, #294).
• [ ] forge.json's check.run names no test file. It runs bun x tsc --noEmit && bun scripts/test-shards.ts --timeout 60000 --gate (or an equivalent the runner reads from scripts/gate.ts). scripts/check.ts still runs the one string in forge.json.
• [ ] test/check-list.test.ts goes red when:

- a test/*.test.ts file is in neither list; - a file is in both lists; - an exclusion has no reason or no issue number; - an entry names a file that does not exist. This file is itself in GATE.

• [ ] On the forge, the pull's check runs the same files main runs today, plus test/check-list.test.ts, and it is green. The pull lists the before and after file sets to show that.
• [ ] README: one paragraph on how a file joins the gate. Move it from EXCLUDED to GATE in a pull whose check is green.

Files

scripts/gate.ts (new), scripts/test-shards.ts (read the list), forge.json, test/check-list.test.ts (new), README.md.

Risks

• Do not touch scripts/shards.ts (#307), and do not edit any red test.
• Do not admit any file in this pull. Admitting files is #261's job once this lands.

Suggested builder: Mason.