📝 Place test-wide suppressions in the config

Drop the fixed no-floating-promises header exception from CONTRIBUTING.md
and AGENTS.md; agents must not add a source disable or edit .oxlintrc.json.
Record the split in tooling.md and testing.md: a one-site false positive is a
source disable, a file-class one lives in the test override.

CONTRIBUTING.md

@@ -155,12 +155,9 @@ reaches for by default:
   [Script prefix convention](#script-prefix-convention).
 - **`oxlint-disable` directives live in source, not `.oxlintrc.json`.** The
   trade-off must sit next to the code it silences. This is a _human_ last-resort
-  convention; agents must not add these — see
-  [AGENTS.md § Never do](./AGENTS.md#never-do). The sole agent exception is the
-  fixed file-level header at the top of a `*.test.ts` file, spelled exactly:
-  `/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync
-type-assertion library that the type-aware linter misidentifies as a promise
-*/` — any other suppression stays human-last-resort. (why:
+  convention; agents must not add one, nor edit `.oxlintrc.json` to silence a
+  finding (e.g. `typescript/no-floating-promises`) — see
+  [AGENTS.md § Never do](./AGENTS.md#never-do). (why:
   [development/tooling.md](./development/tooling.md#oxlint-disable-directives-live-next-to-the-code))
 - **Don't put slow / network / whole-project scans in `check` or pre-commit.**
   Advisory scans are not correctness gates; they belong under `maintain:`. (why: