fix: suppress fallback_model warnings for blank/disabled fields (#373)

When fallback_model is present in config.yaml but has blank fields or
explicitly set enabled: false, treat it as intentionally disabled rather
than emitting startup warnings about missing provider/model fields.

Changes:
- gateway/run.py _load_fallback_model(): return None for empty dicts or
  enabled:false fallback_model entries
- hermes_cli/config.py validate_config_structure(): skip warnings when
  fallback_model has enabled: false; improved hint messages to suggest
  the enabled: false convention

gateway/run.py

@@ -1026,6 +1026,16 @@ class GatewayRunner:
                     cfg = _y.safe_load(_f) or {}
                 fb = cfg.get("fallback_providers") or cfg.get("fallback_model") or None
                 if fb:
+                    # Treat empty dict / disabled fallback as "not configured"
+                    if isinstance(fb, dict):
+                        _enabled = fb.get("enabled")
+                        if _enabled is False or (
+                            isinstance(_enabled, str)
+                            and _enabled.strip().lower() in ("false", "0", "no", "off")
+                        ):
+                            return None
+                        if not fb.get("provider") and not fb.get("model"):
+                            return None
                     return fb
         except Exception:
             pass

hermes_cli/config.py

@@ -1421,6 +1421,7 @@ def validate_config_structure(config: Optional[Dict[str, Any]] = None) -> List["
                     ))
 
     # ── fallback_model must be a top-level dict with provider + model ────
+    # Blank or explicitly disabled fallback is intentional — skip validation.
     fb = config.get("fallback_model")
     if fb is not None:
         if not isinstance(fb, dict):
@@ -1430,20 +1431,29 @@ def validate_config_structure(config: Optional[Dict[str, Any]] = None) -> List["
                 "Change to:\n"
                 "  fallback_model:\n"
                 "    provider: openrouter\n"
-                "    model: anthropic/claude-sonnet-4",
+                "    model: anthropic/claude-sonnet-4\n"
+                "Or disable with:\n"
+                "  fallback_model:\n"
+                "    enabled: false",
             ))
         elif fb:
-            if not fb.get("provider"):
+            # Skip warnings when fallback is explicitly disabled (enabled: false)
+            _enabled = fb.get("enabled")
+            if _enabled is False or (isinstance(_enabled, str) and _enabled.strip().lower() in ("false", "0", "no", "off")):
+                pass  # intentionally disabled — no warnings
+            elif not fb.get("provider"):
                 issues.append(ConfigIssue(
                     "warning",
                     "fallback_model is missing 'provider' field — fallback will be disabled",
-                    "Add: provider: openrouter (or another provider)",
+                    "Add: provider: openrouter (or another provider)\n"
+                    "Or disable with: enabled: false",
                 ))
-            if not fb.get("model"):
+            elif not fb.get("model"):
                 issues.append(ConfigIssue(
                     "warning",
                     "fallback_model is missing 'model' field — fallback will be disabled",
-                    "Add: model: anthropic/claude-sonnet-4 (or another model)",
+                    "Add: model: anthropic/claude-sonnet-4 (or another model)\n"
+                    "Or disable with: enabled: false",
                 ))
 
     # ── Check for fallback_model accidentally nested inside custom_providers ──