[codex] add /import for external agents (#27071)

## Why

External-agent import should be discoverable and deliberate without
blocking startup or claiming the public `codex [PROMPT]` CLI namespace.
The slash command keeps the flow local to the interactive TUI and reuses
the existing app-server import API.

## What changed

- add the user-facing `/import` slash command
- detect external-agent importable items only when the command is
invoked
- run imports through the embedded local app-server
- show start and completion messages, refresh configuration, and block
duplicate imports while one is pending
- reject the flow for unsupported remote and local-daemon sessions

## Validation

- `just test -p codex-tui external_agent_config_migration` (10 passed)
- manually exercised an isolated TUI fixture with existing
external-agent setup and session data using a fresh `CODEX_HOME`
- verified picker customization, plugin and session detection, import
completion, repeated invocation, and imported-session resume context
- the broader `just test -p codex-tui` run passed 2,805 tests, with 2
unrelated guardian feature-flag failures and 4 skipped tests

## Draft follow-ups

- review whether completion messaging should remain attached to the
initiating chat if the user switches chats during an import
- review shutdown semantics for an in-progress background import

## 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 4.** Draft while the lower stack dependencies
are reviewed.

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -398,6 +398,10 @@ impl ChatWidget {
             SlashCommand::Skills => {
                 self.open_skills_menu();
             }
+            SlashCommand::Import => {
+                self.app_event_tx
+                    .send(AppEvent::OpenExternalAgentConfigMigration);
+            }
             SlashCommand::Hooks => {
                 self.add_hooks_output();
             }
@@ -1010,6 +1014,7 @@ impl ChatWidget {
             | SlashCommand::Logout
             | SlashCommand::Mention
             | SlashCommand::Skills
+            | SlashCommand::Import
             | SlashCommand::Hooks
             | SlashCommand::Title
             | SlashCommand::Statusline