Bounce rejected inbox items back to the sender's inbox
reject_inbox previously discarded the stable id, leaving the sender
unaware. Now it parses the source queue out of the inbox key and
re-inboxes the task there with a return key (`<receiver>-<seq>`), so
each round-trip is uniquely identified.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>