Summary:
- Return from external agent import before session history import
finishes
- Run session import work in the background and emit the existing
completion notification when it is done
- Serialize session imports so duplicate requests do not create
duplicate imported threads
Verification:
- cargo test -p codex-app-server external_agent_config_
- cargo test -p codex-external-agent-sessions
- just fix -p codex-app-server
- just fix -p codex-external-agent-sessions
- git diff --check