effortLevel config is not respected when switching models via /model command

### Describe the feature or problem you'd like to solve

_No response_

### Proposed solution

The effortLevel setting in ~/.copilot/config.json is correctly applied when a new session starts. However, when switching models mid-session using the /model command, the effort level resets to "medium" by
default instead of inheriting the value from the config.

Expected behavior: When switching models via /model, the effort level should default to the value set in config.json (effortLevel), so users don't have to manually re-select it every time.

Suggested improvement: Either:

1. Pre-select the configured effortLevel in the /model effort picker UI, or
2. Support per-model default effort levels in config.json (e.g., "models": { "claude-sonnet-4.6": { "effortLevel": "high" } })

Steps to reproduce:

1. Set "effortLevel": "high" in ~/.copilot/config.json
2. Start a new session — effort level is correctly "high" ✅
3. Use /model to switch to another model — effort level resets to "medium" ❌
4. Use /model to switch back — still "medium" ❌

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start at the /model command handling and the loading of ~/.copilot/config.json, then reproduce the switch with effortLevel set to high. Trace where the effort picker defaults during a model change; done means switching models preserves the configured effortLevel instead of resetting to medium.

索引モデルが issue の本文から書いたものです。