## Why
Persisted multi-agent v2 children created after the client disconnects need lifecycle listeners so their runtimes can be released once idle. Listener attachment must also wait for an earlier runtime's cleanup to finish.
## What changed
- Start lifecycle listeners for persisted v2 children even when no initialized clients are connected, and defer attachment until pending unload cleanup completes.
- Keep ephemeral children loaded so follow-up tasks retain their model context.
- Raise the default `thread_unload_delay_secs` from 60 seconds to 1800 seconds (30 minutes), updating the configuration documentation and sample.
## Testing
Add regression coverage for children spawned after disconnect with zero- and one-second unload delays, including waiting for pending cleanup, keeping running threads loaded, releasing idle runtimes, and preserving an ephemeral child's earlier reply across a follow-up. Update configuration tests for the new default and explicit overrides.
GitOrigin-RevId: 2fecc77b5c03237aa8fd7eacc4bd119b99d36e0a
jif
2fdf047c96