Add API routes for workspace file management:
- GET /api/workspace - get workspace info
- POST /api/workspace/files - upload file
- GET /api/workspace/files - list files
- GET /api/workspace/files/{id} - get file info
- GET /api/workspace/files/{id}/download - download file
- DELETE /api/workspace/files/{id} - soft delete
- Stream file content when signed URLs unavailable
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>