## Summary
Adds a simple `/plan` slash command in the TUI that switches the active
collaboration mode to Plan mode. The command is only available when the
`collaboration_modes` feature is enabled.
## Changes
- Add `plan_mask` helper in `codex-rs/tui/src/collaboration_modes.rs`
- Add `SlashCommand::Plan` metadata in
`codex-rs/tui/src/slash_command.rs`
- Implement and hard-gate `/plan` dispatch in
`codex-rs/tui/src/chatwidget.rs`
- Hide `/plan` when collaboration modes are disabled in
`codex-rs/tui/src/bottom_pane/slash_commands.rs`
- Update command popup tests in
`codex-rs/tui/src/bottom_pane/command_popup.rs`
- Add a focused unit test for `/plan` in
`codex-rs/tui/src/chatwidget/tests.rs`
## Behavior notes
- `/plan` is now a no-op if `Feature::CollaborationModes` is disabled.
- When enabled, `/plan` switches directly to Plan mode without opening
the picker.
## Codex author
`codex resume 019c05da-d7c3-7322-ae2c-3ca38d0ef702`
Keep an unmasked base collaboration mode and apply the active mask on
demand. Simplify the TUI mask helpers and update tests/docs to match the
mask contract.
## Summary
- Replace the plan‑implementation prompt with a standard selection
popup.
- “Yes” submits a user turn in Execute via a dedicated app event to
preserve normal transcript behavior.
- “No” simply dismisses the popup.
<img width="977" height="433" alt="Screenshot 2026-01-22 at 2 00 54 PM"
src="https://github.com/user-attachments/assets/91fad06f-7b7a-4cd8-9051-f28a19b750b2"
/>
## Changes
- Add a plan‑implementation popup using `SelectionViewParams`.
- Add `SubmitUserMessageWithMode` so “Yes” routes through
`submit_user_message` (ensures user history + separator state).
- Track `saw_plan_update_this_turn` so the prompt appears even when only
`update_plan` is emitted.
- Suppress the plan popup on replayed turns, when messages are queued,
or when a rate‑limit prompt is pending.
- Add `execute_mode` helper for collaboration modes.
- Add tests for replay/queued/rate‑limit guards and plan update without
final message.
- Add snapshots for both the default and “No”‑selected popup states.
- Only use collaboration modes in the tui state to track model and
effort.
- No behavior change without the collaboration modes flag.
- Change model and effort on /model, /collab (behind a flag), and
shift+tab (behind flag)
Charley Cunningham