feat(CORE-001D1): harden unit-aware workspace persistence

app/models/workspace.py

@@ -26,7 +26,14 @@ class WorkspaceRepository:
     def workspace_exists(self, workspace_id: str) -> bool:
         if not self.is_valid_workspace_id(workspace_id):
             return False
-        return self._portfolio_path(workspace_id).exists()
+        portfolio_path = self._portfolio_path(workspace_id)
+        if not portfolio_path.exists():
+            return False
+        try:
+            PortfolioRepository(portfolio_path).load()
+        except (ValueError, TypeError, FileNotFoundError):
+            return False
+        return True
 
     def create_workspace(self, workspace_id: str | None = None) -> PortfolioConfig:
         resolved_workspace_id = workspace_id or str(uuid4())