Integrate Q in builtin vscode AI/Chat features via provider model

## Problem

[Since vscode 1.99](https://code.visualstudio.com/updates/v1_99#_bring-your-own-key-byok-preview), vscode has a `registerChatModelProvider` interface which allows plugins to provide an AI model, which can be selected in the vscode UI:

For example, https://github.com/isidorn/claude provides the Anthropic model.

- example code: https://github.com/search?q=vscode+registerChatModelProvider+language%3ATypeScript&type=code
- docs: https://code.visualstudio.com/api/extension-guides/language-model

## Expected behavior

Implement the chat / language model provider interface(s). Q should appear in these menus:

- `GitHub Copilot: Change Completions Model`
- 
- `Manage Models`...
- 
- `MCP: Add Server...`
- 

Start with VS Code’s registerChatModelProvider interface and the linked language-model documentation. No repository files or tests are named, so first locate the extension’s Amazon Q integration points and determine how the provider interfaces map to the requested menus. Done means Q appears in the listed completion-model, Manage Models, and MCP server flows.

Written by the indexing model from the issue text.