feat(providers): add native NVIDIA NIM provider

Adds NVIDIA NIM as a first-class provider: ProviderConfig in
auth.py, HermesOverlay in providers.py, curated models
(Nemotron plus other open source models hosted on
build.nvidia.com), URL mapping in model_metadata.py, aliases
(nim, nvidia-nim, build-nvidia, nemotron), and env var tests.

Docs updated: providers page, quickstart table, fallback
providers table, and README provider list.

tests/hermes_cli/test_api_key_providers.py

@@ -33,6 +33,7 @@ class TestProviderRegistry:
         ("huggingface", "Hugging Face", "api_key"),
         ("zai", "Z.AI / GLM", "api_key"),
         ("xai", "xAI", "api_key"),
+        ("nvidia", "NVIDIA NIM", "api_key"),
         ("kimi-coding", "Kimi / Moonshot", "api_key"),
         ("minimax", "MiniMax", "api_key"),
         ("minimax-cn", "MiniMax (China)", "api_key"),
@@ -57,6 +58,12 @@ class TestProviderRegistry:
         assert pconfig.base_url_env_var == "XAI_BASE_URL"
         assert pconfig.inference_base_url == "https://api.x.ai/v1"
 
+    def test_nvidia_env_vars(self):
+        pconfig = PROVIDER_REGISTRY["nvidia"]
+        assert pconfig.api_key_env_vars == ("NVIDIA_API_KEY",)
+        assert pconfig.base_url_env_var == "NVIDIA_BASE_URL"
+        assert pconfig.inference_base_url == "https://integrate.api.nvidia.com/v1"
+
     def test_copilot_env_vars(self):
         pconfig = PROVIDER_REGISTRY["copilot"]
         assert pconfig.api_key_env_vars == ("COPILOT_GITHUB_TOKEN", "GH_TOKEN", "GITHUB_TOKEN")