Copilot Chat Integration surface squad context via @squad

## Description
Integrate with VS Code's Copilot Chat API to surface team context when users mention squad-related topics. High-impact feature that makes SquadUI a context hub for AI-assisted development.

## Approach
- Investigate `vscode.chat` participant API for registering a `@squad` participant
- When invoked, provide team roster, recent decisions, current work status as context
- Start with manual trigger (command or chat participant), plan for deeper integration as APIs mature
- Note: public API surface may be limited design for graceful degradation

## Acceptance Criteria
- [ ] `@squad` chat participant registered in VS Code Copilot Chat
- [ ] Provides team roster context when asked about team members
- [ ] Surfaces recent decisions when asked about project decisions
- [ ] Shows current work status when asked about progress
- [ ] Graceful fallback if chat API is unavailable

## Notes
This is a large feature that may need to be scoped iteratively. The chat participant API surface should be investigated first as a spike.