docs(agents): state the hidden-tests rule as the invariant the gate enforces (#15340)

`AGENTS.md`'s hidden-tests bullet was exclude-shaped, but
`check:type-check-coverage` enforces the invariant per file and
spelling-agnostically: a test named by an `exclude` OR never reached by any
`include` is hidden from the typecheck. The prose declared a narrower rule than
the tree enforces, so an author who obeyed it to the letter could still hide a
whole test tree by never including it.

Restate the bullet as the invariant, name both spellings that break it, and name
`check:type-check-coverage` as the enforcing instrument. Rewritten inside its
existing 6 lines at the existing wrap width (widest line 87B before and after),
so the ratcheted file line count is unchanged at 1161.


Claude-Session: https://claude.ai/code/session_019RfFHiRCSs3JXLK4cwcfox

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]

AGENTS.md

@@ -72,12 +72,12 @@ graduated" to the next author.
 
 Three principles the ratchet's invariants encode, worth knowing before you fight them:
 
-- **Never `exclude` `*.test.ts` / `*.spec.ts` from a package's `tsconfig.json`** —
-  `tsc --noEmit` reads that config, so the exclusion hides the tests from the very
-  check the `typecheck` script advertises (a green gate over source nothing read). When
-  the build config must keep the exclusion, add a sibling `tsconfig.test.json` and name
-  it in the `typecheck` script (the `packages/spec` pattern); the sibling may carry its
-  own *module* semantics to match vitest, never its own *strictness*.
+- **A package's `tsconfig.json` reaches every `*.test.ts` / `*.spec.ts` in it** —
+  `tsc --noEmit` reads that config, so a test it misses is hidden from the very check
+  the `typecheck` script advertises (a green gate over source nothing read).
+  `check:type-check-coverage` fails both spellings per file: an `exclude` naming it, an
+  `include` that never does. A sibling `tsconfig.test.json` named in the `typecheck`
+  script may carry its own *module* semantics for vitest, never its own *strictness*.
 - **A `@ts-expect-error` in a file no tsc program compiles is a phantom check** — it
   evaluates never, and deleting it leaves every gate just as green. Before writing
   one, check the file is compiled. Test-layer residue lives in the per-file,