♻️ Drop the last duplicated prose from development/

The GitHub Flow description and the type-first enforcement sentence still
appeared in both CONTRIBUTING.md and development/. development/ now carries only
the context and rationale that is not in CONTRIBUTING.md.

development/testing.md

@@ -31,14 +31,6 @@ written before the runtime assertion, and both before the implementation.
 - Testing the type only: it would not catch handler wiring, `exhaustive()`
   throwing, or the `otherwise` fallback (see `src/index.test.ts`).
 
-## Enforcement
-
-Type-first is also enforced structurally: `npm test` runs `check:tsc` before the
-test runner, so a wrong type can never be papered over by a passing assertion.
-Per [AGENTS.md § Never do](../AGENTS.md#never-do), reach green honestly — fix the
-types so both the type check and the runtime assertion pass, never suppress the
-ones you can't make pass.
-
 The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
 listed in
 [CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).