fix(gateway): skip model auto-detection for custom/local providers

Mirrors the CLI fix for the gateway /model handler. When the user is on
a custom provider (provider=custom, localhost, or 127.0.0.1 endpoint),
/model <name> no longer tries to auto-detect a provider switch.

Previously, typing /model openrouter/nvidia/nemotron:free on Telegram
while on a localhost endpoint would silently accept the model name on
the local server — auto-detection failed to match the free model, so
the provider stayed as custom with the localhost base_url. The user saw
'Model changed' but requests still went to localhost, which doesn't
serve that model.

Now shows the endpoint URL and provider:model syntax tip, matching
the CLI behavior.

gateway/run.py

@@ -2498,8 +2498,22 @@ class GatewayRunner:
 
         # Parse provider:model syntax
         target_provider, new_model = parse_model_input(args, current_provider)
+
+        # Detect custom/local provider — skip auto-detection to prevent
+        # silently accepting an OpenRouter model name on a localhost endpoint.
+        # Users must use explicit provider:model syntax to switch away.
+        _resolved_base = ""
+        try:
+            from hermes_cli.runtime_provider import resolve_runtime_provider as _rtp
+            _resolved_base = _rtp(requested=current_provider).get("base_url", "")
+        except Exception:
+            pass
+        is_custom = current_provider == "custom" or (
+            "localhost" in _resolved_base or "127.0.0.1" in _resolved_base
+        )
+
         # Auto-detect provider when no explicit provider:model syntax was used
-        if target_provider == current_provider:
+        if target_provider == current_provider and not is_custom:
             from hermes_cli.models import detect_provider_for_model
             detected = detect_provider_for_model(new_model, current_provider)
             if detected:
@@ -2580,7 +2594,18 @@ class GatewayRunner:
         # Clear fallback state since user explicitly chose a model
         self._effective_model = None
         self._effective_provider = None
-        return f"🤖 Model changed to `{new_model}` ({persist_note}){provider_note}{warning}\n_(takes effect on next message)_"
+
+        # Helpful hint when staying on a custom/local endpoint
+        custom_hint = ""
+        if is_custom and not provider_changed:
+            endpoint = _resolved_base or "custom endpoint"
+            custom_hint = (
+                f"\n**Endpoint:** `{endpoint}`"
+                "\n_To switch providers, use_ `/model provider:model`"
+                "\n_e.g._ `/model openrouter:anthropic/claude-sonnet-4`"
+            )
+
+        return f"🤖 Model changed to `{new_model}` ({persist_note}){provider_note}{warning}{custom_hint}\n_(takes effect on next message)_"
 
     async def _handle_provider_command(self, event: MessageEvent) -> str:
         """Handle /provider command - show available providers."""