♻️ Single-home the actionable rules and the rationale

development/ had restated the actionables that CONTRIBUTING.md owns: the
branching step list, the script prefix list, the feedback-tier rule of thumb,
the commit convention and the type-driven test loop. Those now live only in
CONTRIBUTING.md; development/ keeps the decision blocks and links to the rule.
development/README.md, CONTRIBUTING.md and AGENTS.md state the 'write each fact
once' principle explicitly.

development/testing.md

@@ -8,26 +8,9 @@ the way it is.
 
 ## Type-driven development
 
-New behavior follows **type-driven development** (in Edwin Brady's sense):
-_treat the type as the plan for a program, and use the compiler and type checker
-as your assistant, guiding you to a complete program that satisfies the type_
-([idris-lang.org](https://www.idris-lang.org/)). Here that plan is the
-`expectTypeOf` assertion, written first. The loop is **type → red → green →
-refactor**:
-
-1. **Type** — write the compile-time expectation first
-   (`expectTypeOf(...).toEqualTypeOf<…>()`) and let `npm run check:tsc` fail on
-   the _type_. The type error is the spec you want to hit before the runtime
-   logic exists.
-2. **Red** — add the matching runtime assertion (`assert.*`) so
-   `npm run test:unit` now fails on behavior.
-3. **Green** — implement in `src/*.ts` until both the type check and the test
-   pass.
-4. **Refactor** — with the type system and the tests as the safety net, then
-   `npm run verify` as the definition-of-done gate.
-
-This is why every test in the suite pairs an `expectTypeOf(...)` with an
-`assert.*` — keep them together.
+The rules — the loop and the pairing rule — are in
+[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
+What follows is why the loop is type-driven and what was rejected.
 
 #### Decision (2026-09)
 
@@ -56,18 +39,14 @@ Per [AGENTS.md § Never do](../AGENTS.md#never-do), reach green honestly — fix
 types so both the type check and the runtime assertion pass, never suppress the
 ones you can't make pass.
 
-## Test tiers
+The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
+listed in
+[CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
+`c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
+build step. The runner relies on the `.ts` import-extension convention (see
+[tooling.md](./tooling.md#source-imports-use-ts-extensions)).
 
-- `npm run test:unit` — the test runner alone, for the fast local loop.
-- `npm test` — `check:tsc` + the unit suite; this is what pre-push and the
-  baseline check run.
-- `npm run test:ci` — adds c8 coverage; used by CI. `c8` uses V8 coverage, so
-  the `--strip-types` source is instrumented without a build step.
-
-The runner is `node --test --strip-types "src/**/*.test.ts"`. It relies on the
-`.ts` import-extension convention (see [tooling.md](./tooling.md#source-imports-use-ts-extensions)).
-
-#### Known issue
+## Known issues
 
 - The type-aware linter misidentifies `expectTypeOf()` as a floating promise,
   so `src/index.test.ts` carries a file-level `oxlint-disable