feat: config structure validation — detect malformed YAML at startup (#5426)

Add validate_config_structure() that catches common config.yaml mistakes:
- custom_providers as dict instead of list (missing '-' in YAML)
- fallback_model accidentally nested inside another section
- custom_providers entries missing required fields (name, base_url)
- Missing model section when custom_providers is configured
- Root-level keys that look like misplaced custom_providers fields

Surface these diagnostics at three levels:
1. Startup: print_config_warnings() runs at CLI and gateway module load,
   so users see issues before hitting cryptic errors
2. Error time: 'Unknown provider' errors in auth.py and model_switch.py
   now include config diagnostics with fix suggestions
3. Doctor: 'hermes doctor' shows a Config Structure section with all
   issues and fix hints

Also adds a warning log in runtime_provider.py when custom_providers
is a dict (previously returned None silently).

Motivated by a Discord user who had malformed custom_providers YAML
and got only 'Unknown Provider' with no guidance on what was wrong.

17 new tests covering all validation paths.

hermes_cli/model_switch.py

@@ -419,14 +419,25 @@ def switch_model(
         # Resolve the provider
         pdef = resolve_provider_full(explicit_provider, user_providers)
         if pdef is None:
+            _switch_err = (
+                f"Unknown provider '{explicit_provider}'. "
+                f"Check 'hermes model' for available providers, or define it "
+                f"in config.yaml under 'providers:'."
+            )
+            # Check for common config issues that cause provider resolution failures
+            try:
+                from hermes_cli.config import validate_config_structure
+                _cfg_issues = validate_config_structure()
+                if _cfg_issues:
+                    _switch_err += "\n\nRun 'hermes doctor' — config issues detected:"
+                    for _ci in _cfg_issues[:3]:
+                        _switch_err += f"\n  • {_ci.message}"
+            except Exception:
+                pass
             return ModelSwitchResult(
                 success=False,
                 is_global=is_global,
-                error_message=(
-                    f"Unknown provider '{explicit_provider}'. "
-                    f"Check 'hermes model' for available providers, or define it "
-                    f"in config.yaml under 'providers:'."
-                ),
+                error_message=_switch_err,
             )
 
         target_provider = pdef.id