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

tests/test_api_key_providers.py

@@ -704,14 +704,14 @@ class TestHasAnyProviderConfigured:
         assert _has_any_provider_configured() is True
 
     def test_config_dict_no_provider_no_creds_still_false(self, monkeypatch, tmp_path):
-        """config.yaml model dict with only 'default' key and no creds stays false."""
+        """config.yaml model dict with empty default and no creds stays false."""
         import yaml
         from hermes_cli import config as config_module
         hermes_home = tmp_path / ".hermes"
         hermes_home.mkdir()
         config_file = hermes_home / "config.yaml"
         config_file.write_text(yaml.dump({
-            "model": {"default": "anthropic/claude-opus-4.6"},
+            "model": {"default": ""},
         }))
         monkeypatch.setattr(config_module, "get_env_path", lambda: hermes_home / ".env")
         monkeypatch.setattr(config_module, "get_hermes_home", lambda: hermes_home)