Improve Plan mode reasoning selection flow (#12303)

Addresses https://github.com/openai/codex/issues/11013

## Summary
- add a Plan implementation path in the TUI that lets users choose
reasoning before switching to Default mode and implementing
- add Plan-mode reasoning scope handling (Plan-only override vs
all-modes default), including config/schema/docs plumbing for
`plan_mode_reasoning_effort`
- remove the hardcoded Plan preset medium default and make the reasoning
popup reflect the active Plan override as `(current)`
- split the collaboration-mode switch notification UI hint into #12307
to keep this diff focused

If I have `plan_mode_reasoning_effort = "medium"` set in my
`config.toml`:
<img width="699" height="127" alt="Screenshot 2026-02-20 at 6 59 37 PM"
src="https://github.com/user-attachments/assets/b33abf04-6b7a-49ed-b2e9-d24b99795369"
/>

If I don't have `plan_mode_reasoning_effort` set in my `config.toml`:
<img width="704" height="129" alt="Screenshot 2026-02-20 at 7 01 51 PM"
src="https://github.com/user-attachments/assets/88a086d4-d2f1-49c7-8be4-f6f0c0fa1b8d"
/>

## Codex author
`codex resume 019c78a2-726b-7fe3-adac-3fa4523dcc2a`

codex-rs/tui/src/app_event.rs

@@ -164,6 +164,12 @@ pub(crate) enum AppEvent {
         model: ModelPreset,
     },
 
+    /// Open the Plan-mode reasoning scope prompt for the selected model/effort.
+    OpenPlanReasoningScopePrompt {
+        model: String,
+        effort: Option<ReasoningEffort>,
+    },
+
     /// Open the full model picker (non-auto models).
     OpenAllModelsPopup {
         models: Vec<ModelPreset>,
@@ -258,6 +264,9 @@ pub(crate) enum AppEvent {
     /// Update whether the rate limit switch prompt has been acknowledged for the session.
     UpdateRateLimitSwitchPromptHidden(bool),
 
+    /// Update the Plan-mode-specific reasoning effort in memory.
+    UpdatePlanModeReasoningEffort(Option<ReasoningEffort>),
+
     /// Persist the acknowledgement flag for the full access warning prompt.
     PersistFullAccessWarningAcknowledged,
 
@@ -268,6 +277,9 @@ pub(crate) enum AppEvent {
     /// Persist the acknowledgement flag for the rate limit switch prompt.
     PersistRateLimitSwitchPromptHidden,
 
+    /// Persist the Plan-mode-specific reasoning effort.
+    PersistPlanModeReasoningEffort(Option<ReasoningEffort>),
+
     /// Persist the acknowledgement flag for the model migration prompt.
     PersistModelMigrationPromptAcknowledged {
         from_model: String,