fix(config): refuse to write when config.yaml has YAML syntax errors

set_config_value() and unset_config_value() silently replaced the
entire config with an empty dict when config.yaml could not be
parsed. A single YAML syntax error would cause 'hermes config set'
to wipe all settings and write only the new key. Now exits with
error and preserves the existing file.

hermes_cli/config.py

@@ -4837,8 +4837,15 @@ def set_config_value(key: str, value: str, force: bool = False):
         try:
             with open(config_path, encoding="utf-8") as f:
                 user_config = fast_safe_load(f) or {}
-        except Exception:
-            user_config = {}
+        except Exception as exc:
+            print(
+                f"✗ Cannot parse {config_path}: {exc}\n"
+                f"  The file contains a YAML syntax error. Fix the error\n"
+                f"  in your config file first, then retry.\n"
+                f"  (hermes config edit will open it in your editor.)",
+                file=sys.stderr,
+            )
+            sys.exit(1)
     
     # Handle nested keys (e.g., "tts.provider") including numeric list
     # indices (e.g., "custom_providers.0.api_key").  Delegates to
@@ -5045,8 +5052,15 @@ def unset_config_value(key: str):
         try:
             with open(config_path, encoding="utf-8") as f:
                 user_config = fast_safe_load(f) or {}
-        except Exception:
-            user_config = {}
+        except Exception as exc:
+            print(
+                f"✗ Cannot parse {config_path}: {exc}\n"
+                f"  The file contains a YAML syntax error. Fix the error\n"
+                f"  in your config file first, then retry.\n"
+                f"  (hermes config edit will open it in your editor.)",
+                file=sys.stderr,
+            )
+            sys.exit(1)
 
     removed = _unset_nested(user_config, key)