Add stale issue and PR management workflow

[PAMulligan]

Description

Add a GitHub Actions workflow that automatically labels and closes stale issues and pull requests after a period of inactivity.

Why

As the project grows and attracts community contributions, stale issues and PRs accumulate. Automated management keeps the issue tracker healthy and signals to contributors that the project is actively maintained.

Acceptance Criteria

• Create .github/workflows/stale.yml using actions/stale
• Mark issues as stale after 60 days of inactivity
• Close stale issues after 14 more days with no response
• Mark PRs as stale after 30 days of inactivity
• Close stale PRs after 7 more days with no response
• Exempt issues with priority: high and help wanted labels
• Add a stale label (gray color) for the workflow to use
• Add friendly stale/close messages

Example

name: Stale Issues
on:
  schedule:
    - cron: '0 0 * * 1'  # Weekly on Monday
jobs:
  stale:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/stale@v9
        with:
          stale-issue-message: >
            This issue has been automatically marked as stale due to
            60 days of inactivity. It will be closed in 14 days if
            no further activity occurs.
          days-before-stale: 60
          days-before-close: 14
          exempt-issue-labels: 'priority: high,help wanted'