Official subagent uses --no-session, so T3 could project cards but could
not open the child. Inject a T3-owned override that persists --session,
reports sessionFile, and binds each result as a child thread. Follow-up
sends allocate a new RPC and resume with switch_session.

Duplicate subagent registrations abort Pi, so spawn disables extension
discovery and drops the official tool from launchArgs.