feat: add ESLint, lint-staged, and update pre-commit hook + CI

- Add ESLint 9 flat config (eslint.config.mjs) with typescript-eslint
  recommended rules and eslint-config-prettier
- Add lint-staged to run eslint+prettier only on staged files
- Update pre-commit hook to use lint-staged instead of full prettier check
- Add `lint` and `format:check` scripts to package.json
- Add Lint step to CI workflow
- Fix resulting lint errors: unused vars (_ctx, _options, catch binding),
  any→unknown in type declarations, stale eslint-disable comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -21,7 +21,10 @@ jobs:
       - run: npm ci
 
       - name: Format check
-        run: npx prettier --check '**/*.{js,ts,css,json,md}'
+        run: npm run format:check
+
+      - name: Lint
+        run: npm run lint
 
       - name: Type check
         run: npm run typecheck