* fix: read run stop_reason from runtime context
* fix: address review feedback for #4188 stop_reason integration
- migration 0005: use safe_add_column for consistency and drift detection
- worker: clear runtime.context stop_reason at start of each _stream_once
turn so a clean continuation doesn't inherit a prior cap reason
- tests: replace circular unit test with real middleware integration
tests that exercise LoopDetectionMiddleware._apply and
TokenBudgetMiddleware._apply through the worker, proving the full
middleware → runtime.context → persist pipeline
* fix(test): resume conftest
* fix: stamp stop_reason in all guard middlewares, fix clearing semantics
