- TaskRunner.run() and WorkflowRunner.run()/resume() accept an optional
stop_reason callable alongside should_stop, distinguishing a genuine
cancellation from other stop conditions (e.g. lost lease).
- is_cancellation_reason() shared helper added to runtime/task.py.
- WorkflowRunner._stop_status() now branches cancelled/failed based on
stop_reason, correcting a prior blanket cancelled-on-any-stop behavior
that conflicted with the host-agent-protocol spec's requirement to
distinguish cancellation from lease-loss stops.
- Default behavior (stop_reason=None) is preserved exactly for both
runners so existing callers/tests are unaffected.
- Task 1 of openspec change task-cancellation.
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