fix: remove hardcoded OpenRouter/opus defaults

No model, base_url, or provider is assumed when the user hasn't
configured one.  Previously the defaults dict in cli.py, AIAgent
constructor args, and several fallback paths all hardcoded
anthropic/claude-opus-4.6 + openrouter.ai/api/v1 — silently routing
unconfigured users to OpenRouter, which 404s for anyone using a
different provider.

Now empty defaults force the setup wizard to run, and existing users
who already completed setup are unaffected (their config.yaml has
the model they chose).

Files changed:
- cli.py: defaults dict, _DEFAULT_CONFIG_MODEL
- run_agent.py: AIAgent.__init__ defaults, main() defaults
- hermes_cli/config.py: DEFAULT_CONFIG
- hermes_cli/runtime_provider.py: is_fallback sentinel
- acp_adapter/session.py: default_model
- tests: updated to reflect empty defaults

acp_adapter/session.py

@@ -426,7 +426,7 @@ class SessionManager:
 
         config = load_config()
         model_cfg = config.get("model")
-        default_model = "anthropic/claude-opus-4.6"
+        default_model = ""
         config_provider = None
         if isinstance(model_cfg, dict):
             default_model = str(model_cfg.get("default") or default_model)