Add explicit permissions to guidance-comment workflow job

[Copilot]

Adds explicit permissions declaration to the guidance-comment job in the PR quality workflow to follow the principle of least privilege.

Changes

• Added permissions: pull-requests: write to guidance-comment job in .github/workflows/pr-quality.yml
• Restricts the job to only the permission it requires rather than inheriting broader default workflow permissions

guidance-comment:
  if: github.event.action == 'opened'
  runs-on: ubuntu-latest
  permissions:
    pull-requests: write
  steps:
    - name: Post QA Guidance Comment
      uses: peter-evans/create-or-update-comment@v3
      ...

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