fix(delegate): default inherit_mcp_toolsets=true, drop version bump

Follow-up on helix4u's PR #14211:
- Flip default to true: narrowing toolsets=['web','browser'] expresses
  'I want these extras', not 'silently strip MCP'. Parent MCP tools
  (registered at runtime) should survive narrowing by default.
- Drop _config_version bump (22->23); additive nested key under
  delegation.* is handled by _deep_merge, no migration needed.
- Update tests to reflect new default behavior.

tests/tools/test_delegate.py

@@ -1059,35 +1059,7 @@ class TestChildCredentialPoolResolution(unittest.TestCase):
             self.assertEqual(mock_child._credential_pool, mock_pool)
 
     @patch("tools.delegate_tool._load_config", return_value={})
-    def test_build_child_agent_does_not_preserve_mcp_toolsets_by_default(self, mock_cfg):
-        parent = _make_mock_parent()
-        parent.enabled_toolsets = ["web", "browser", "mcp-MiniMax"]
-
-        with patch("run_agent.AIAgent") as MockAgent:
-            mock_child = MagicMock()
-            MockAgent.return_value = mock_child
-
-            _build_child_agent(
-                task_index=0,
-                goal="Test narrowed toolsets",
-                context=None,
-                toolsets=["web", "browser"],
-                model=None,
-                max_iterations=10,
-                parent_agent=parent,
-                task_count=1,
-            )
-
-        self.assertEqual(
-            MockAgent.call_args[1]["enabled_toolsets"],
-            ["web", "browser"],
-        )
-
-    @patch(
-        "tools.delegate_tool._load_config",
-        return_value={"inherit_mcp_toolsets": True},
-    )
-    def test_build_child_agent_can_preserve_parent_mcp_toolsets(self, mock_cfg):
+    def test_build_child_agent_preserves_mcp_toolsets_by_default(self, mock_cfg):
         parent = _make_mock_parent()
         parent.enabled_toolsets = ["web", "browser", "mcp-MiniMax"]
 
@@ -1111,6 +1083,34 @@ class TestChildCredentialPoolResolution(unittest.TestCase):
             ["web", "browser", "mcp-MiniMax"],
         )
 
+    @patch(
+        "tools.delegate_tool._load_config",
+        return_value={"inherit_mcp_toolsets": False},
+    )
+    def test_build_child_agent_strict_intersection_when_opted_out(self, mock_cfg):
+        parent = _make_mock_parent()
+        parent.enabled_toolsets = ["web", "browser", "mcp-MiniMax"]
+
+        with patch("run_agent.AIAgent") as MockAgent:
+            mock_child = MagicMock()
+            MockAgent.return_value = mock_child
+
+            _build_child_agent(
+                task_index=0,
+                goal="Test narrowed toolsets",
+                context=None,
+                toolsets=["web", "browser"],
+                model=None,
+                max_iterations=10,
+                parent_agent=parent,
+                task_count=1,
+            )
+
+        self.assertEqual(
+            MockAgent.call_args[1]["enabled_toolsets"],
+            ["web", "browser"],
+        )
+
 
 class TestChildCredentialLeasing(unittest.TestCase):
     def test_run_single_child_acquires_and_releases_lease(self):