🔧 Replace ESLint and Prettier with oxlint and oxfmt

- Drop .eslintrc.cjs and .prettierrc, add .oxlintrc.json and .oxfmtrc.json
- oxlint covers correctness/suspicious/perf/style/restriction categories
  plus import and typescript plugins; oxfmt reads the same .prettierrc-style
  options we had before
- Update lefthook.yml pre-commit jobs to run oxlint/oxfmt instead of
  eslint/prettier
- Update .vscode/extensions.json (oxc replaces eslint/prettier/vitest) and
  settings.json (oxc becomes the default formatter for TS/JS)
- Add 'oxlint', 'oxfmt', 'oxc', 'nodenext' to cspell dictionary
- Rewrite README 'Development' section to describe the new toolchain

project-specs.md

@@ -288,6 +288,7 @@ Resolves #...
   - `dbaeumer.vscode-eslint` (ESLint)
   - `streetsidesoftware.code-spell-checker` (cspell)
   - `vitest.explorer` (for test integration)
+  - TODO: Native test extension with node test runner
   - `ms-vscode.vscode-typescript-next` (for latest TS features, optional)
 
 - Add `.vscode/tasks.json` for common tasks (optional):