feat(providers): add native xAI provider

Adds xAI as a first-class provider: ProviderConfig in auth.py,
HermesOverlay in providers.py, 11 curated Grok models, URL mapping
in model_metadata.py, aliases (x-ai, x.ai), and env var tests.
Uses standard OpenAI-compatible chat completions.

Closes #7050

hermes_cli/providers.py

@@ -127,6 +127,11 @@ HERMES_OVERLAYS: Dict[str, HermesOverlay] = {
         is_aggregator=True,
         base_url_env_var="HF_BASE_URL",
     ),
+    "xai": HermesOverlay(
+        transport="openai_chat",
+        base_url_override="https://api.x.ai/v1",
+        base_url_env_var="XAI_BASE_URL",
+    ),
 }
 
 
@@ -163,6 +168,10 @@ ALIASES: Dict[str, str] = {
     "z.ai": "zai",
     "zhipu": "zai",
 
+    # xai
+    "x-ai": "xai",
+    "x.ai": "xai",
+
     # kimi-for-coding (models.dev ID)
     "kimi": "kimi-for-coding",
     "kimi-coding": "kimi-for-coding",
@@ -341,6 +350,7 @@ def get_label(provider_id: str) -> str:
 
 
 
+
 def is_aggregator(provider: str) -> bool:
     """Return True when the provider is a multi-model aggregator."""
     pdef = get_provider(provider)