/model picker is inconsistent: model step is arrow-keys-only, but the effort/context steps accept direct number input

### Summary
The `/model` flow uses two different interaction patterns across its consecutive steps. Selecting the **model** requires arrow-key navigation, but the **subsequent** prompts (reasoning effort, context, etc.) are numbered and accept direct numeric entry. This forces a hand/keyboard-mode switch mid-flow and is surprising.

### Steps to reproduce
1. Run `/model`.
2. **Step 1 — model list:** all models are listed. The only way to move the selection is the arrow keys; typing `1`, `2`, … does nothing.
3. Pick a model with arrows and press Enter.
4. **Step 2+ — effort / context / etc.:** these options are numbered, so you can immediately press `1` / `2` to choose — no arrows needed.

### Expected
Consistent selection across all steps of `/model`. Either:
- the model list also supports direct numeric quick-select (preferred — faster for long lists), **or**
- all steps use the same single mechanism.

### Actual
Step 1 is arrow-only; later steps are numbered/typeable. The interaction model changes partway through the same command.

### Environment
- Copilot CLI: `1.0.60`
- OS: Windows 11 Pro
- Terminal: WezTerm
- Shell: PowerShell 7.6.2

### Suggested fix
Add numeric quick-select to the model list to match the effort/context pickers (keep arrows working too). This keeps long model lists fast while making the whole `/model` flow consistent.

Start by running the `/model` command and comparing the model-list interaction with the numbered effort and context pickers. Trace the `/model` entry point and the existing picker behavior; done means model choices accept numeric quick-select while arrow navigation still works and the interaction is consistent across all steps.

索引モデルが issue の本文から書いたものです。