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.

CONTRIBUTING.md

@@ -20,10 +20,28 @@ theme/          # Custom theme overrides
 
 ## Content Guidelines
 
-- Write content in both English (British) and Simplified Chinese.
+- Write content in both English (British) and Simplified Chinese. British
+  spelling is required throughout — interface text, documentation, code
+  comments and commit messages (`customised`, `centre`, `colour`, and so on).
 - Blog posts and documentation articles are licensed under CC-BY-4.0.
 - Code contributions are licensed under MIT.
 
+## Navigation and Sidebars
+
+Navigation and sidebar structure live in JSON files beside the content, not in
+the theme:
+
+- `_nav.json` sets the top navigation. Its `text` values are i18n keys resolved
+  through `i18n.json`, so the `en-gb` and `zh-hans` copies stay identical. Add
+  new keys to `i18n.json` for **both** locales — a key present for only one
+  locale fails the build.
+- `_meta.json` sets sidebar grouping and ordering within a directory. Supplying
+  one replaces the generated listing for that directory, so every page you want
+  shown must be listed — anything omitted disappears from the sidebar silently.
+
+Do not add a `_meta.json` next to `_nav.json` at the root of a locale
+directory; doing so collapses the whole site into one flat sidebar.
+
 ## Pull Request Process
 
 1. Ensure the site builds cleanly: `pnpm build`