feat: simple fallback model for provider resilience

When the primary model/provider fails after retries (rate limit, overload,
auth errors, connection failures), Hermes automatically switches to a
configured fallback model for the remainder of the session.

Config (in ~/.hermes/config.yaml):

  fallback_model:
    provider: openrouter
    model: anthropic/claude-sonnet-4

Supports all major providers: OpenRouter, OpenAI, Nous, DeepSeek, Together,
Groq, Fireworks, Mistral, Gemini — plus custom endpoints via base_url and
api_key_env overrides.

Design principles:
- Dead simple: one fallback model, not a chain
- One-shot: switches once, doesn't ping-pong back
- Zero new dependencies: uses existing OpenAI client
- Minimal code: ~100 lines in run_agent.py, ~5 lines in cli.py/gateway
- Three trigger points: max retries exhausted, non-retryable client errors,
  and invalid response exhaustion

Does NOT trigger on context overflow or payload-too-large errors (those
are handled by the existing compression system).

Addresses #737.

25 new tests, 2492 total passing.

gateway/run.py

@@ -194,6 +194,7 @@ class GatewayRunner:
         self._ephemeral_system_prompt = self._load_ephemeral_system_prompt()
         self._reasoning_config = self._load_reasoning_config()
         self._provider_routing = self._load_provider_routing()
+        self._fallback_model = self._load_fallback_model()
 
         # Wire process registry into session store for reset protection
         from tools.process_registry import process_registry
@@ -393,6 +394,26 @@ class GatewayRunner:
             pass
         return {}
 
+    @staticmethod
+    def _load_fallback_model() -> dict | None:
+        """Load fallback model config from config.yaml.
+
+        Returns a dict with 'provider' and 'model' keys, or None if
+        not configured / both fields empty.
+        """
+        try:
+            import yaml as _y
+            cfg_path = _hermes_home / "config.yaml"
+            if cfg_path.exists():
+                with open(cfg_path) as _f:
+                    cfg = _y.safe_load(_f) or {}
+                fb = cfg.get("fallback_model", {}) or {}
+                if fb.get("provider") and fb.get("model"):
+                    return fb
+        except Exception:
+            pass
+        return None
+
     async def start(self) -> bool:
         """
         Start the gateway and all configured platform adapters.
@@ -2623,6 +2644,7 @@ class GatewayRunner:
                 platform=platform_key,
                 honcho_session_key=session_key,
                 session_db=self._session_db,
+                fallback_model=self._fallback_model,
             )
             
             # Store agent reference for interrupt support