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.

tests/gateway/test_model_command_custom_providers.py

@@ -0,0 +1,61 @@
+"""Regression tests for gateway /model support of config.yaml custom_providers."""
+
+import yaml
+import pytest
+
+from gateway.config import Platform
+from gateway.platforms.base import MessageEvent, MessageType
+from gateway.run import GatewayRunner
+from gateway.session import SessionSource
+
+
+def _make_runner():
+    runner = object.__new__(GatewayRunner)
+    runner.adapters = {}
+    return runner
+
+
+def _make_event(text="/model"):
+    return MessageEvent(
+        text=text,
+        message_type=MessageType.TEXT,
+        source=SessionSource(platform=Platform.TELEGRAM, chat_id="12345", chat_type="dm"),
+    )
+
+
+@pytest.mark.asyncio
+async def test_handle_model_command_lists_saved_custom_provider(tmp_path, monkeypatch):
+    hermes_home = tmp_path / ".hermes"
+    hermes_home.mkdir()
+    (hermes_home / "config.yaml").write_text(
+        yaml.safe_dump(
+            {
+                "model": {
+                    "default": "gpt-5.4",
+                    "provider": "openai-codex",
+                    "base_url": "https://chatgpt.com/backend-api/codex",
+                },
+                "providers": {},
+                "custom_providers": [
+                    {
+                        "name": "Local (127.0.0.1:4141)",
+                        "base_url": "http://127.0.0.1:4141/v1",
+                        "model": "rotator-openrouter-coding",
+                    }
+                ],
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    import gateway.run as gateway_run
+
+    monkeypatch.setattr(gateway_run, "_hermes_home", hermes_home)
+    monkeypatch.setattr("agent.models_dev.fetch_models_dev", lambda: {})
+
+    result = await _make_runner()._handle_model_command(_make_event())
+
+    assert result is not None
+    assert "Local (127.0.0.1:4141)" in result
+    assert "custom:local-(127.0.0.1:4141)" in result
+    assert "rotator-openrouter-coding" in result