ci: add stale issue and PR cleanup workflow

[kietcoderlor]

Summary

Closes #35.

This PR adds .github/workflows/stale.yml using actions/stale@v9 to help manage inactive issues and pull requests.

What changed

• Added a scheduled stale cleanup workflow.
• Issues are marked as stale after 60 days of inactivity.
• Stale issues are closed after 14 additional days of inactivity.
• Pull requests are marked as stale after 30 days of inactivity.
• Pull requests are not automatically closed because the issue only requested stale labeling for PRs.
• Issues and PRs labeled pinned or security are exempt.
• The stale label is removed when new activity occurs.
• Added workflow_dispatch so maintainers can run the workflow manually.

Test plan

• Confirmed the diff only touches .github/workflows/stale.yml.
• Ran git diff --check.
• Ran npx -y prettier --check .github/workflows/stale.yml.
• Verified the workflow uses actions/stale@v9.
• Verified issue stale/close timing matches the issue requirements.
• Verified PR stale timing matches the issue requirements.
• Verified PR auto-close is disabled with days-before-pr-close: -1.
• Verified pinned and security labels are exempt.