Add Plan mode model override

[aaudin90]

Motivation

Plan mode already has a dedicated reasoning-effort override, but it still follows the global model unless the user changes the global default. This makes it awkward to keep a separate model for planning while using another model for implementation.

This adds a plan_mode_model config option so Plan mode can keep its own model override without changing the global model selection.

Summary

• Add plan_mode_model to top-level config and profile config parsing.
• Include plan_mode_model in generated config schema and user config docs.
• Apply the Plan-mode model override when entering Plan mode and when submitting a Plan-mode message.
• Persist Plan-mode-only model choices through the existing config edit path.
• Update the Plan-mode model/reasoning scope prompt so users can choose whether a model settings change applies only to Plan mode or to all modes.
• Add coverage for profile precedence, config edits, Plan-mode submission behavior, prompt actions, and prompt copy.

Testing

• cargo test -p codex-tui plan_reasoning_scope_popup_mentions

Full workspace tests were not run locally.