feat: change auto_fetch default to true

Auto-fetching missing dependencies from upstream is the more useful default
behavior. Users who need fast, network-free resolution can explicitly set
auto_fetch=false.

Artifacts are content-addressed by SHA256, so reproducibility concerns don't
apply - the same version always produces the same artifact.

backend/tests/test_dependencies.py

@@ -1073,17 +1073,17 @@ class TestAutoFetchDependencies:
     """Tests for auto-fetch functionality in dependency resolution.
 
     These tests verify:
-    - Resolution with auto_fetch=false (default) behavior is unchanged
-    - Resolution with auto_fetch=true attempts to fetch missing dependencies
+    - Resolution with auto_fetch=true (default) fetches missing dependencies from upstream
+    - Resolution with auto_fetch=false skips network calls for fast resolution
     - Proper handling of missing/non-existent packages
     - Response schema includes fetched artifacts list
     """
 
     @pytest.mark.integration
-    def test_resolve_auto_fetch_false_is_default(
+    def test_resolve_auto_fetch_true_is_default(
         self, integration_client, test_package, unique_test_id
     ):
-        """Test that auto_fetch=false is the default and behaves as before."""
+        """Test that auto_fetch=true is the default (no fetch needed when all deps cached)."""
         project_name, package_name = test_package
 
         # Upload a simple artifact without dependencies