## What changed
- Add stable `sub_error_type` values for external-agent session import failures,
including detection, preparation, configuration, thread storage, and ledger
update errors.
- Preserve these values through import results and analytics while leaving
unrelated plugin and memory errors unchanged.
## Testing
- Verify that a missing session reports `session_not_detected` in both the
completed import result and its analytics event.
GitOrigin-RevId: cbf1f52df4bb1ac107442c4afa0b130fa32ad0a5
## What changed
- Move migration detection, import orchestration, models, and reporting from
`codex-app-server` into `codex-external-agent-migration`.
- Keep app-server protocol conversion in a dedicated boundary module.
- Organize source-specific configuration, sessions, hooks, plugins, commands,
subagents, and memory handling behind shared migration adapters.
- Relocate and split the migration tests alongside the extracted service.
GitOrigin-RevId: 3378ec2064806a28136d6ac98664ca1d75414a4e
charlesgong-openai