fix(config): widen empty-section guard to _deep_merge in load_config

Sibling site of the load_cli_config fix (#58277): _deep_merge treated a
YAML-null section (terminal: with no value) as an override, replacing
the entire DEFAULT_CONFIG dict for that section with None. Every
downstream consumer expecting a mapping was a latent crash, and default
sub-keys were silently lost. A None override of a dict default is now
ignored, matching the CLI loader's behavior. Scalar-null overrides are
unchanged.

hermes_cli/config.py

@@ -6243,6 +6243,12 @@ def _deep_merge(base: dict, override: dict) -> dict:
     Keys in *override* take precedence. If both values are dicts the merge
     recurses, so a user who overrides only ``tts.elevenlabs.voice_id`` will
     keep the default ``tts.elevenlabs.model_id`` intact.
+
+    An empty section key in config.yaml (``terminal:`` with no value) parses
+    as YAML ``None``; treating that as an override would replace the entire
+    default dict with ``None`` and crash every downstream consumer that
+    expects a mapping (#58277). A ``None`` override of a dict default is
+    ignored — same as the key being absent.
     """
     result = base.copy()
     for key, value in override.items():
@@ -6252,6 +6258,8 @@ def _deep_merge(base: dict, override: dict) -> dict:
             and isinstance(value, dict)
         ):
             result[key] = _deep_merge(result[key], value)
+        elif key in result and isinstance(result[key], dict) and value is None:
+            continue
         else:
             result[key] = value
     return result

tests/hermes_cli/test_config.py

@@ -360,6 +360,34 @@ class TestLoadConfigParseFailure:
             assert capsys.readouterr().err == ""
 
 
+class TestEmptyConfigSections:
+    """Empty section keys (``terminal:`` with no value) parse as YAML None
+    and must not replace the default dict for that section (#58277)."""
+
+    def test_null_section_keeps_defaults_in_load_config(self, tmp_path):
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            (tmp_path / "config.yaml").write_text(
+                "model:\n  default: test/custom\n"
+                "terminal:\n"
+                "display:\n"
+            )
+            config = load_config()
+            assert config["model"]["default"] == "test/custom"
+            assert isinstance(config["terminal"], dict)
+            assert config["terminal"] == DEFAULT_CONFIG["terminal"]
+            assert isinstance(config["display"], dict)
+
+    def test_null_override_of_non_dict_default_still_applies(self, tmp_path):
+        """None only shields dict defaults — explicit null for a scalar
+        key remains an override (unchanged behavior)."""
+        from hermes_cli.config import _deep_merge
+
+        merged = _deep_merge({"scalar": 5, "section": {"a": 1}},
+                             {"scalar": None, "section": None})
+        assert merged["scalar"] is None
+        assert merged["section"] == {"a": 1}
+
+
 class TestSaveAndLoadRoundtrip:
     @staticmethod
     def _deny_config_reads(config_path):