Multi-Workspace Dashboard unified view for multi-squad orgs

## Description
Support organizations running multiple Squad teams across different repositories. Provide a unified dashboard view that aggregates team status, decisions, and metrics across workspaces.

## Approach
- Detect multi-root workspaces with multiple `.ai-team/` folders
- Aggregate dashboard data across workspaces
- Workspace selector or merged view with workspace labels
- Consider VS Code multi-root workspace API

## Acceptance Criteria
- [ ] Detect multiple squad-enabled workspaces
- [ ] Dashboard aggregates or switches between workspaces
- [ ] Each workspace's team is clearly identified
- [ ] Tree views support multi-workspace navigation

## Notes
This is a large architectural feature. Consider starting with a workspace selector dropdown before building full aggregation.