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/config.py

@@ -429,6 +429,20 @@ OPTIONAL_ENV_VARS = {
         "category": "provider",
         "advanced": True,
     },
+    "DEEPSEEK_API_KEY": {
+        "description": "DeepSeek API key for direct DeepSeek access",
+        "prompt": "DeepSeek API Key",
+        "url": "https://platform.deepseek.com/api_keys",
+        "password": True,
+        "category": "provider",
+    },
+    "DEEPSEEK_BASE_URL": {
+        "description": "Custom DeepSeek API base URL (advanced)",
+        "prompt": "DeepSeek Base URL",
+        "url": "",
+        "password": False,
+        "category": "provider",
+    },
 
     # ── Tool API keys ──
     "FIRECRAWL_API_KEY": {