feat: validate /model against live API instead of hardcoded lists

Replace the static catalog-based model validation with a live API probe.
The /model command now hits the provider's /models endpoint to check if
the requested model actually exists:

- Model found in API → accepted + saved to config
- Model NOT found in API → rejected with 'Error: not a valid model'
  and fuzzy-match suggestions from the live model list
- API unreachable → graceful fallback to hardcoded catalog (session-only
  for unrecognized models)
- Format errors (empty, spaces, missing '/') still caught instantly
  without a network call

The API probe takes ~0.2s for OpenRouter (346 models) and works with any
OpenAI-compatible endpoint (Ollama, vLLM, custom, etc.).

32 tests covering all paths: format checks, API found, API not found,
API unreachable fallback, CLI integration.

cli.py

@@ -2079,6 +2079,7 @@ class HermesCLI:
                     validation = validate_requested_model(
                         new_model,
                         provider_for_validation,
+                        api_key=self.api_key,
                         base_url=self.base_url,
                     )
                 except Exception: