Move external agent config out of core

[pakrym-oai]

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