ci: add cleanup-branches workflow (dry-run)

[Logiraptor]

Adds the cleanup-branches workflow from shared-workflows in dry-run mode.

This will run weekly (Monday 9am UTC) and print which branches would be deleted without actually deleting them. Check the workflow run output to verify, then set dry-run: false to enable.

Branches are eligible for cleanup if they:

• Are not in an open PR
• Are not protected
• Have not been updated in the last 3 months

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}