📝 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/tooling.md

@@ -106,26 +106,34 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`
 
 #### Decision (2026-09)
 
-A type-aware rule that false-positives is silenced with a source-level
-`oxlint-disable` directive (see `src/primitive.ts`,
-`src/primitive.test.ts`), not by turning the rule off in `.oxlintrc.json`.
+A type-aware rule that false-positives **at one site** is silenced with a
+source-level `oxlint-disable` directive (see `src/primitive.ts`). A rule that is
+wrong for a whole **file class** is turned off in a `.oxlintrc.json` `overrides`
+entry instead — e.g. `typescript/no-floating-promises` (synchronous
+`expectTypeOf` reads as an unhandled promise) and `unicorn/no-null` (intentional
+`null` inputs) for `**/*.test.ts`. The same exemption is not repeated as a
+file-level header in every affected file.
 
 #### Why
 
-- The disable sits next to the code it silences, visible to anyone reading the
-  source.
-- The rule stays on everywhere else, so only the mis-firing line is exempted.
+- A one-site disable sits next to the code it silences, visible to anyone
+  reading the source, and the rule stays on everywhere else.
+- A file-class rule is a property of the file class, not of one line; the
+  override states it once, where the rest of the file-class config lives.
 
 #### Rejected
 
-- A project-wide disable in `.oxlintrc.json` for a false positive: it hides the
-  exemption from the reader of the affected code and switches the rule off
-  repo-wide for a one-site problem.
+- A project-wide disable in `.oxlintrc.json` for a one-site false positive: it
+  hides the exemption from the reader of the affected code and switches the rule
+  off repo-wide for a one-site problem.
+- A repeated file-level `oxlint-disable` header for a file-class false positive:
+  the copies drift and scatter one config decision across the tree.
 
 #### Known issue
 
-- A source-level disable is a _human_ last resort. AI agents must not add one;
-  they fix the type at its root (see [AGENTS.md § Never do](../AGENTS.md#never-do)).
+- Both placements are _human_ last resorts. AI agents must neither add a source
+  disable nor edit `.oxlintrc.json`; they fix the type at its root (see
+  [AGENTS.md § Never do](../AGENTS.md#never-do)).
 
 ### Unwanted stylistic rules are turned off in the config