Add CI task for shadow branch auto-rebase and release status

[zclawz]

Summary

Add CI coverage/automation for the shadow branch so it stays rebased on new upstream tips and failures are visible.

Requirements

• Automatically rebase shadow when new tips land.
• Fail visibly if the rebase fails.
• Include shadow rebase success/failure status in DevNet release Telegram messages.

Notes

This should make shadow branch drift obvious during DevNet release flow instead of silently lagging or requiring manual checks.