Add storage layer for workspace files:
- WorkspaceStorageBackend: abstract interface
- GCSWorkspaceStorage: Google Cloud Storage implementation
- LocalWorkspaceStorage: local filesystem for self-hosted
- WorkspaceManager: high-level file operations with session scoping
- Session-scoped virtual paths: /sessions/{session_id}/{filename}
- Fallback to API proxy when GCS signed URLs unavailable
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>