Replace for-await stream consumption in watchChildAgent with
waitForIdle() + onClose() callbacks on AsyncAgent. This prevents
conflict with Hub.consumeAgent() which also reads the Channel.
Add shutdownSubagentRegistry() for clean Hub shutdown, guard
Hub access with isHubInitialized(), and clean resumedRuns in sweep.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Implement subagent registry, persistence store, and announcement flow
for child agent lifecycle management. Includes types, registry with
auto-archive sweeper, session JSONL reading for result extraction,
and formatted announcement message delivery.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
