Enable checkstyle reporting in CI workflow

[Copilot]

The checkstyle GitHub Action was commented out and previously failed with "Resource not accessible by integration" due to missing authentication token.

Changes

• Uncommented lint step that generates checkstyle XML report via ESLint
• Uncommented checkstyle upload action that posts violations as PR annotations
• Added explicit token: ${{ secrets.GITHUB_TOKEN }} parameter to fix authentication error

- name: Upload checkstyle report
  if: matrix.node_version == env.TARGET_COVERAGE_NODE_VERSION
  uses: jwgmeligmeyling/checkstyle-github-action@master
  with:
    path: '**/checkstyle-result.xml'
    token: ${{ secrets.GITHUB_TOKEN }}

The workflow already has required permissions (checks: write, pull-requests: write). The explicit token ensures the action can create check runs with annotations.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support checkstyle</issue_title>
<issue_description>### Issue

Run jwgmeligmeyling/checkstyle-github-action@master
With the provided path, there will be 1 results uploaded
Creating annotations for /home/runner/work/pinpoint-node-agent/pinpoint-node-agent/checkstyle-result.xml
Uploading 31 / 31 annotations to GitHub as Checkstyle with conclusion failure
Error: HttpError: Resource not accessible by integration
```</issue_description>

<agent_instructions>You have to add checkstyle report into Pull requests.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Support checkstyle #385

---

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[feelform]

You have to use secrets.REPO_TOKEN token

[feelform]

LCOV of commit a626eee during CI #1298

Summary coverage rate:
  lines......: 84.4% (3522 of 4173 lines)
  functions..: 82.3% (849 of 1031 functions)
  branches...: 70.6% (1117 of 1583 branches)

Files changed coverage rate: n/a