📝 Add development/ docs for decisions and known issues

Category files under development/ replace the decision prose that was
scattered through README.md and CONTRIBUTING.md. Each decision is a block with
Decision (YYYY-MM) / Why / Rejected / Known issue; the new library.md records
the public API contract and its limitations. AGENTS.md and backlog.tasks now
point at the new home.

AGENTS.md

@@ -64,5 +64,6 @@ In both cases, follow [CONTRIBUTING.md § Testing discipline (type-driven)](./CO
 - [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).
-- [README.md § Tooling decisions](./README.md#tooling-decisions) — the rationale behind each tool choice; read before changing tooling.
+- [development/](./development/README.md) — the decisions, rejected alternatives and known issues behind the rules; the “why” that CONTRIBUTING.md links to. Read the relevant file before changing an area.
+- [development/tooling.md](./development/tooling.md) — the rationale behind each tool choice; read before changing tooling.
 - [package.json `#scripts`](./package.json) — the source of truth for every command (the `LEFTHOOK_FILES` convention scopes them to staged files vs. the whole project).