Add GitHub Actions workflows to match caldera core

[deacon-mp]

Summary

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

Workflows added:

• security.yml: Runs security checks (safety and bandit) via tox on Python 3.13, triggered on push and pull request events.
• stale.yml: Automatically marks issues and pull requests as stale after 60 days of inactivity, and closes them after another 60 days.
• greetings.yml: Greets first-time contributors when they open an issue or pull request.

All actions are pinned to specific SHAs for security.