Add GitHub Actions workflows to match caldera core

[deacon-mp]

Summary

This PR updates and adds GitHub Actions workflows to match the caldera core repository.

Changes:

• testing.yml (updated): Replaced outdated workflow that used @v2 actions and Python 3.8 only. Now uses pinned SHA actions and tests against Python 3.10, 3.11, 3.12, and 3.13.
• stale.yml (new): Automatically marks issues and pull requests as stale after 60 days of inactivity, and closes them after another 60 days.
• greetings.yml (new): Greets first-time contributors when they open an issue or pull request.

All actions are pinned to specific SHAs for security.