## Why
The external-agent import picker is easier to review when its rendering
refactor lands separately from new state and interaction behavior. This
layer is intended to be behavior-neutral.
## What changed
- extract external-agent migration rendering into a dedicated `render`
module
- preserve existing behavior while separating presentation from
interaction logic
- establish a smaller foundation for the import picker UX in the next PR
## Validation
- `just test -p codex-tui external_agent_config_migration` (10 passed)
## Stack
1. [#27064](https://github.com/openai/codex/pull/27064): remove the
startup migration flow
2. [#27065](https://github.com/openai/codex/pull/27065): extract the
picker renderer
3. [#27070](https://github.com/openai/codex/pull/27070): add the
external-agent import picker UX
4. [#27071](https://github.com/openai/codex/pull/27071): expose the flow
through `/import`
**This PR is stack item 2.** Draft while the lower stack dependency is
reviewed.
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