Preserve background task notifications after end_turn

[Gujiassh]

Summary

• keep the s08_background_tasks.py loop alive until background task results are delivered instead of exiting immediately on end_turn
• mirror the same behavior in s_full.py so the capstone agent does not drop background completions either
• add regression tests covering both entrypoints

Testing

• python3 -m unittest tests/test_background_notifications.py -v
• python3 -m py_compile agents/s08_background_tasks.py agents/s_full.py tests/test_background_notifications.py

Closes #46

[vercel]

@Gujiassh is attempting to deploy a commit to the crazyboym's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Gujiassh]

The only visible blocker on this PR right now looks like the Vercel team authorization check, rather than a code/test regression. If preview deploys matter here, someone on the team will need to authorize it; otherwise I’m happy for maintainers to ignore that status and review the patch on its own merits.