fix: restore fork PR lint/format CI workflow (#9846)

## Problem

The lint/format CI workflow was broken for fork PRs in two ways:

### 1. Node version mismatch in setup-frontend action
The `setup-frontend` shared action (created in #8377) was missed when
Node version was standardized to `.nvmrc` in #9521. It still used
`node-version: 'lts/*'` instead of `node-version-file: '.nvmrc'`.

### 2. Fork PRs with lint issues silently passed CI
Fork PRs with auto-fixable lint/format issues got a **green checkmark**
despite having unfixed issues:
1. Auto-fix steps (`lint:fix`, `format`) fix issues in the workspace
2. `Commit changes` is correctly skipped for forks (can't push to fork
branches)
3. `Final validation` passes because it runs on the already-fixed
workspace
4. The `Comment on PR about manual fix needed` step tries to post a
comment via `actions/github-script`, but fork PRs have a read-only
`GITHUB_TOKEN` — the comment silently fails (`continue-on-error: true`)
5. **Result**: workflow reports success, contributor thinks their code
is clean

## Fix

- **setup-frontend**: Use `node-version-file: '.nvmrc'` instead of
`node-version: 'lts/*'`
- **ci-lint-format**: Replace the broken fork comment step with an
explicit `exit 1` that fails CI and prints clear fix instructions in the
log. This follows the principle from `.github/AGENTS.md`: fork PRs can't
post comments, so don't try.

## Testing
- [ ] Verify fork PRs with clean code still pass
- [ ] Verify fork PRs with lint issues now properly fail (instead of
silently passing)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9846-fix-restore-fork-PR-lint-format-CI-workflow-3226d73d3650811cb5bfe9f1f989cc0c)
by [Unito](https://www.unito.io)

.github/actions/setup-frontend/action.yaml

@@ -19,7 +19,7 @@ runs:
     - name: Setup Node.js
       uses: actions/setup-node@v6
       with:
-        node-version: 'lts/*'
+        node-version-file: '.nvmrc'
         cache: 'pnpm'
         cache-dependency-path: './pnpm-lock.yaml'