🔧 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

.vscode/extensions.json

@@ -1,9 +1,7 @@
 {
     "recommendations": [
-        "esbenp.prettier-vscode",
-        "dbaeumer.vscode-eslint",
+        "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
-        "vitest.explorer",
         "ms-vscode.vscode-typescript-next"
     ]
-}
+}