Add CI workflow to notify and auto-close stale PRs

Add a GitHub Actions workflow (actions/stale) that marks a pull request as stale after a period of inactivity and posts a notification comment, then automatically closes it if it stays inactive. Only PRs are processed; issues are excluded. Runs on a schedule and supports manual dispatch.

JIRA Issue: BA-6526

Start with the GitHub Actions workflow configuration and the actions/stale documentation. Configure scheduled and manual runs to process only pull requests, post a stale notification after inactivity, and close PRs that remain inactive. Done means the workflow runs in both modes and excludes issues.

Written by the indexing model from the issue text.