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/core/config.schema.json

@@ -467,6 +467,9 @@
         "personality": {
           "$ref": "#/definitions/Personality"
         },
+        "plan_mode_reasoning_effort": {
+          "$ref": "#/definitions/ReasoningEffort"
+        },
         "sandbox_mode": {
           "$ref": "#/definitions/SandboxMode"
         },
@@ -1901,6 +1904,9 @@
       ],
       "description": "Optionally specify a personality for the model"
     },
+    "plan_mode_reasoning_effort": {
+      "$ref": "#/definitions/ReasoningEffort"
+    },
     "profile": {
       "description": "Profile to use from the `profiles` map.",
       "type": "string"