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`

docs/config.md

@@ -32,4 +32,13 @@ The generated JSON Schema for `config.toml` lives at `codex-rs/core/config.schem
 
 Codex stores "do not show again" flags for some UI prompts under the `[notice]` table.
 
+## Plan mode defaults
+
+`plan_mode_reasoning_effort` lets you set a Plan-mode-specific default reasoning
+effort override. When unset, Plan mode uses the built-in Plan preset default
+(currently `medium`). When explicitly set (including `none`), it overrides the
+Plan preset. The string value `none` means "no reasoning" (an explicit Plan
+override), not "inherit the global default". There is currently no separate
+config value for "follow the global default in Plan mode".
+
 Ctrl+C/Ctrl+D quitting uses a ~1 second double-press hint (`ctrl + c again to quit`).