fix: include custom_providers in /model command listings and resolution

Custom providers defined in config.yaml under  were
completely invisible to the /model command in both gateway (Telegram,
Discord, etc.) and CLI. The provider listing skipped them and explicit
switching via --provider failed with "Unknown provider".

Root cause: gateway/run.py, cli.py, and model_switch.py only read the
 dict from config, ignoring  entirely.

Changes:
- providers.py: add resolve_custom_provider() and extend
  resolve_provider_full() to check custom_providers after user_providers
- model_switch.py: propagate custom_providers through switch_model(),
  list_authenticated_providers(), and get_authenticated_provider_slugs();
  add custom provider section to provider listings
- gateway/run.py: read custom_providers from config, pass to all
  model-switch calls
- cli.py: hoist config loading, pass custom_providers to listing and
  switch calls

Tests: 4 new regression tests covering listing, resolution, and gateway
command handler. All 71 tests pass.

cli.py

@@ -4130,6 +4130,16 @@ class HermesCLI:
         # Parse --provider and --global flags
         model_input, explicit_provider, persist_global = parse_model_flags(raw_args)
 
+        user_provs = None
+        custom_provs = None
+        try:
+            from hermes_cli.config import load_config
+            cfg = load_config()
+            user_provs = cfg.get("providers")
+            custom_provs = cfg.get("custom_providers")
+        except Exception:
+            pass
+
         # No args at all: show available providers + models
         if not model_input and not explicit_provider:
             model_display = self.model or "unknown"
@@ -4139,18 +4149,10 @@ class HermesCLI:
 
             # Show authenticated providers with top models
             try:
-                # Load user providers from config
-                user_provs = None
-                try:
-                    from hermes_cli.config import load_config
-                    cfg = load_config()
-                    user_provs = cfg.get("providers")
-                except Exception:
-                    pass
-
                 providers = list_authenticated_providers(
                     current_provider=self.provider or "",
                     user_providers=user_provs,
+                    custom_providers=custom_provs,
                     max_models=6,
                 )
                 if providers:
@@ -4191,6 +4193,8 @@ class HermesCLI:
             current_api_key=self.api_key or "",
             is_global=persist_global,
             explicit_provider=explicit_provider,
+            user_providers=user_provs,
+            custom_providers=custom_provs,
         )
 
         if not result.success: