feat: auto-detect provider when switching models via /model (#1506)

When typing /model deepseek-chat while on a different provider, the
model name now auto-resolves to the correct provider instead of
silently staying on the wrong one and causing API errors.

Detection priority:
1. Direct provider with credentials (e.g. DEEPSEEK_API_KEY set)
2. OpenRouter catalog match with proper slug remapping
3. Direct provider without creds (clear error beats silent failure)

Also adds DeepSeek as a first-class API-key provider — just set
DEEPSEEK_API_KEY and /model deepseek-chat routes directly.

Bare model names get remapped to proper OpenRouter slugs:
  /model gpt-5.4 → openai/gpt-5.4
  /model claude-opus-4.6 → anthropic/claude-opus-4.6

Salvages the concept from PR #1177 by @virtaava with credential
awareness and OpenRouter slug mapping added.

Co-authored-by: virtaava <virtaava@users.noreply.github.com>

hermes_cli/auth.py

@@ -147,6 +147,14 @@ PROVIDER_REGISTRY: Dict[str, ProviderConfig] = {
         api_key_env_vars=("MINIMAX_CN_API_KEY",),
         base_url_env_var="MINIMAX_CN_BASE_URL",
     ),
+    "deepseek": ProviderConfig(
+        id="deepseek",
+        name="DeepSeek",
+        auth_type="api_key",
+        inference_base_url="https://api.deepseek.com/v1",
+        api_key_env_vars=("DEEPSEEK_API_KEY",),
+        base_url_env_var="DEEPSEEK_BASE_URL",
+    ),
 }