active_agents_except (finish_scan, wait_for_message) treats budget_paused as
active, so a root cannot finish the scan over a parked child. park_for_budget
only transitions a running agent, and the wake back to running happens under
the coordinator lock.
Adds budget_policy: stop | pause to run_strix_scan / ReportUsageHooks /
AgentCoordinator, independent of interactive mode. Under pause the agents
get no budget warnings and no sub-agent reserve; each agent parks before
its next LLM call once spent >= limit or the scan is paused, sessions and
sandbox stay alive, and coordinator.resume_budget(max_budget_usd=...)
replaces the limit and wakes every parked agent without adding anything
to any session. coordinator.pause_budget() parks a running scan the same
way. In-flight calls are never cancelled, so spent may end above the
limit. Parked agents count as active for stop_agent.
