## Summary
- Move external agent config migration logic and tests from `codex-core`
into `app-server/src/config`.
- Keep the migration service crate-private to app-server and update the
API adapter imports.
- Remove stale core re-exports and expose only the needed marketplace
source helper.
## Testing
- `cargo test -p codex-app-server config::external_agent_config`
- `just fmt`
- `just fix -p codex-app-server`
- `just fix -p codex-core`
- `git diff --check`
