Fix lost background task results after end_turn

[stablegenius49]

Summary

• keep the s08 background-task loop alive until a running task either finishes or no completions remain to inject
• wake the loop with a condition variable when background threads enqueue results
• add regression tests covering both the wait path and the loop re-entry path
• add tests/test_unit.py so the repo's CI entrypoint actually runs unittest discovery

Testing

• python3 -m unittest tests.test_s08_background_tasks
• python3 tests/test_unit.py
• python3 -m compileall agents/s08_background_tasks.py tests/test_s08_background_tasks.py tests/test_unit.py
• npm ci && npm run build (in web/)

Closes #46

[vercel]

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