Add stale repository identifier action

[matofeder]

This MR adds stale repository identifier action
It implements the repository activity monitoring requirements outlined in the Sovereign Cloud Stack Procedural Standard (SCS-0006) https://github.com/SovereignCloudStack/standards/blob/main/Standards/scs-0006-v1-organization-management.md.

Functionality

• Identifies repositories within the SovereignCloudStack organization that have been inactive for 335 or 365 days
• Reports stale repositories based on recent pushes, releases, and pull requests
• Automatically creates or updates a GitHub issue with the stale repository report
• Runs monthly (on the 1st at 01:00 UTC) and can be manually triggered

Limitations

• The action does not exclude auto-generated PRs (e.g., Renovate, Dependabot)
• It does not handle repositories with no remaining codeowners as stale

TODO: These limitations must be implemented via post-processing or contributed upstream.

Stale repository report generated by the action: #332