fix: workspace creation with stale directories and missing bind mount

- workspace_manager.py: remove stale workspace directories before cloning
  to prevent 'already exists' errors from previous failed attempts
- workspaces.py: add ValueError -> 400 handling, keep 409 for duplicates
- test_tool_instances_legacy.py: fix broken patches for new helpers
  (get_container_name removed, _ensure_backend_network_in_compose added,
  workspace_id/ssh_key_ids mock attributes added)
- docker-compose.traefik.yml: add /data/working-copies bind mount

Quality gates: pytest (19 passed, 1 skipped)

docker-compose.traefik.yml

@@ -13,7 +13,11 @@ services:
     volumes:
       - postgres_data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-headquarter} -d ${POSTGRES_DB:-headquarter}"]
+      test:
+        [
+          "CMD-SHELL",
+          "pg_isready -U ${POSTGRES_USER:-headquarter} -d ${POSTGRES_DB:-headquarter}",
+        ]
       interval: 10s
       timeout: 5s
       retries: 5