Add provider-based grouping (accordion) to model picker for better model discovery

# Feature Request: GitHub Copilot Model Picker Provider Grouping

## Summary
Request to improve the GitHub Copilot model picker UI by adding provider-based grouping (accordion/submenu) to reduce cognitive load and improve discoverability of AI models.

## Problem
Current model picker displays all models in a flat, ungrouped list. As the number of providers and models grows, users experience:
- Difficulty scanning for models from specific providers
- Increased cognitive load remembering which provider offers which model
- Poor scalability with growing model catalog

## Proposed Solution
Implement provider-based grouping using an accordion-style interface:
- Each provider (OpenAI, Anthropic, Google, NVIDIA, etc.) as a collapsible section
- Section headers show provider name and model count
- Click header to expand/collapse provider's models
- Visual indicators for expand/collapse state
- Smooth animations for better UX

## Benefits
- **Faster model selection**: Users can quickly navigate to their preferred provider
- **Reduced cognitive load**: Provider context is immediately visible
- **Better scalability**: Interface remains usable as more providers/models are added
- **Improved discoverability**: Logical grouping matches how users conceptualize AI models

## Mockup Reference
See `model-picker-mockup.html` for visual implementation example showing:
- Provider sections with expand/collapse icons
- Model lists that slide open/closed using transform/scale (layout-performance safe)
- Selection states and hover effects
- Responsive width suitable for dropdown/popup context

## Implementation Notes
- Uses opacity and transform for animations (avoids layout thrashing)
- System font stack for native platform feel
- Keyboard accessible design (expand/collapse via Enter/Space)
- Selection persistence considerations

## Suggested Next Steps
1. Design review with GitHub Copilot UX team
2. Technical feasibility assessment
3. Prototyping in Copilot extension codebase
4. User testing with developer audience
5. A/B testing against current flat-list implementation

## Files
- `github-copilot-model-picker-improvement.md`: Detailed design plan
- `model-picker-mockup.html`: Visual mockup with working interactions
- This file: Executive summary for stakeholder consumption

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*Submitted via GitHub Copilot feedback channel*
*Date: 2026-08-09*

Mockup & full plan gist: https://gist.github.com/mcfuryamen/1e194890e70e444d90d605126da04951

Start by reviewing model-picker-mockup.html and github-copilot-model-picker-improvement.md, then assess how the proposed provider grouping fits the Copilot extension codebase. Done means a feasible design for collapsible provider sections, model counts, accessible keyboard interaction, animation, selection persistence, and validation through the proposed UX and user-testing steps.

索引モデルが issue の本文から書いたものです。