fix: custom endpoint provider shows as openrouter in gateway

Three issues caused the gateway to display 'openrouter' instead of
'Custom endpoint' when users configured a custom OAI-compatible endpoint:

1. hermes setup: custom endpoint path saved OPENAI_BASE_URL and
   OPENAI_API_KEY to .env but never wrote model.provider to config.yaml.
   All other providers (Codex, z.ai, Kimi, etc.) call
   _update_config_for_provider() which sets this — custom was the only
   path that skipped it. Now writes model.provider='custom' and
   model.base_url to config.yaml.

2. hermes model: custom endpoint set model.provider='auto' in config.yaml.
   The CLI display had a hack to detect OPENAI_BASE_URL and override to
   'custom', but the gateway didn't. Now sets model.provider='custom'
   directly.

3. gateway /model and /provider commands: defaulted to 'openrouter' and
   read config.yaml — which had no provider set. Added OPENAI_BASE_URL
   detection fallback (same pattern the CLI uses) as a defensive catch
   for existing users who set up before this fix.

hermes_cli/main.py

@@ -1050,7 +1050,7 @@ def _model_flow_custom(config):
         cfg = load_config()
         model = cfg.get("model")
         if isinstance(model, dict):
-            model["provider"] = "auto"
+            model["provider"] = "custom"
             model["base_url"] = effective_url
         save_config(cfg)
         deactivate_provider()