chore: pin toolchain and add type checking

- Pin TypeScript as a direct dependency. It was only present transitively
  via typescript-eslint, so nothing enforced tsconfig.json and nine
  latent type errors had gone unnoticed.
- Add `pnpm typecheck` (`tsc --noEmit`) and document it.
- Add .npmrc with engine-strict, so the declared Node and pnpm ranges
  are actually enforced.
- Ignore doc_build/ in ESLint. It was unignored and reported 1650 errors
  from build output, which made lint unusable as a gate.
- Refresh README, CLAUDE.md, AGENTS.md and CONTRIBUTING.md to match the
  current structure, build targets and navigation rules.

.npmrc

@@ -0,0 +1 @@
+engine-strict=true