Offer/auto-switch to Auto model when exiting plan mode

## Summary

When exiting plan mode, offer to (or automatically) switch the session to the **Auto** model for implementation. Today, exit-plan-mode prompts let you choose how to execute (e.g., interactive vs. autopilot), but they always stay on whatever model you used to plan. Adding "switch to Auto" as either a configurable default or an explicit exit-plan-mode option would close a high-value cost-optimization workflow.

## Motivation

A common workflow that delivers real cost savings:

1. **Plan** with a strong reasoning model of the user's choice (e.g., Opus, GPT‑5.x, Sonnet at high reasoning) to think through a non-trivial task.
2. **Implement** the resulting plan with the **Auto** model, which is cheaper to run *and* currently carries a 10% discount on top of that.

The plan is the expensive thinking; execution against a well-formed plan is much more mechanical and Auto handles it well. Doing the whole task on a premium model burns tokens unnecessarily, and doing the whole task on Auto often produces a weaker plan. The plan-on-premium → implement-on-Auto split is a sweet spot.

Right now the only way to get this behavior is to:
- Exit plan mode,
- Manually open the model picker,
- Switch to Auto,
- Re-prompt the agent to start implementing.

That's enough friction that most users will just stay on whatever model they planned with, which leaves the savings on the table.

## Proposed behavior

Two flavors, either or both would be great:

### 1. An option in the exit-plan-mode prompt

When the agent surfaces the existing "exit plan mode and implement using …" prompt, add a variant such as:

- *Exit plan mode and implement using **Auto** (recommended for cost)*
- *Exit plan mode and implement using **Auto** in autopilot*

…alongside the current options. Selecting it would atomically switch the model to Auto and continue execution.

### 2. A user setting / feature flag

A config toggle like:

- `planMode.switchToAutoOnExit: true` (or similar)
- Per-project override allowed

When enabled, exiting plan mode automatically switches the active session's model to Auto before execution starts. Users who don't want this behavior just leave the toggle off and see today's behavior.

## Scope

This would most naturally live in the **GitHub Copilot CLI** (this repo) since that's where plan mode and the exit-plan-mode prompt are surfaced, but the same UX would be valuable in the **GitHub Copilot desktop app** sessions that have plan mode. Ideally the setting and the prompt option are consistent across both surfaces.

## Why this is worth doing

- **Real $$ savings** for users running plans frequently — Auto is cheaper *and* discounted.
- **Encodes a best-practice workflow** that power users already use manually.
- **Low-friction**: a single extra option in an already-existing prompt, or a one-line setting.
- **Opt-in**: as a toggle/option it doesn't change behavior for anyone who doesn't want it.

## Out of scope / non-goals

- Not asking to change the *default* model selection.
- Not asking to auto-switch models mid-execution outside the plan→implement transition.
- Not asking to remove any existing exit-plan-mode options.

Happy to elaborate or help test if useful.