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

hermes_cli/config.py

@@ -196,7 +196,7 @@ def ensure_hermes_home():
 # =============================================================================
 
 DEFAULT_CONFIG = {
-    "model": "anthropic/claude-opus-4.6",
+    "model": "",
     "fallback_providers": [],
     "credential_pool_strategies": {},
     "toolsets": ["hermes-cli"],