📝 Document type-first (red/green/blue) testing discipline

New behavior is written type-first: the expectTypeOf (blue) comes
before the assert (red), since for a pattern-matching library the
types are the feature. Adds a CONTRIBUTING.md section and a
discoverability pointer in AGENTS.md's "Read these" index.

Human-facing rationale lives in CONTRIBUTING.md; AGENTS.md only links
it so agents and reviewers don't diverge.

AGENTS.md

@@ -35,6 +35,7 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 ## Read these
 
 - [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) — the constraints the linters don't catch; CI/review bounce these. **The most important section.**
+- [CONTRIBUTING.md § Testing discipline (type-first)](./CONTRIBUTING.md#testing-discipline-type-first) — write the `expectTypeOf` (blue) before the `assert` (red); the types are the feature.
 - [CONTRIBUTING.md § Script prefix convention](./CONTRIBUTING.md#script-prefix-convention) — adding an `npm run` script? reuse an existing prefix or it doesn't belong.
 - [CONTRIBUTING.md § Commit messages](./CONTRIBUTING.md#commit-messages) — gitmoji + imperative + 50/72.
 - [CONTRIBUTING.md § Feedback tiers](./CONTRIBUTING.md#feedback-tiers) — what runs when and at what cost (`watch` / pre-commit / pre-push / `check` / `verify` / `fix` / `maintain` / CI).