📝 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.

development/testing.md

@@ -167,8 +167,8 @@ the CLI is for manual inspection.
 
 ## Known issues
 
-- The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
-  test files that use it (`src/primitive.test.ts`) carry a file-level
-  `oxlint-disable typescript/no-floating-promises` with an explanatory comment.
-  It is a known false positive, not a rule worth disabling project-wide (see
+- The type-aware linter misidentifies `expectTypeOf()` as a floating promise.
+  It is a known false positive, so `typescript/no-floating-promises` is off for
+  `**/*.test.ts` in the `.oxlintrc.json` override rather than repeated as a
+  file-level header (see
   [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).