feat(01-02): provider YAML schema structs with validation and reference YAML files

- Provider, Pattern, VerifySpec, RegistryStats structs in schema.go
- UnmarshalYAML validates format_version >= 1 and last_verified non-empty
- Three reference YAML files: openai, anthropic, huggingface

providers/huggingface.yaml

@@ -0,0 +1,19 @@
+format_version: 1
+name: huggingface
+display_name: HuggingFace
+tier: 3
+last_verified: "2026-04-04"
+keywords:
+  - "hf_"
+  - "huggingface"
+patterns:
+  - regex: 'hf_[A-Za-z0-9]{34,}'
+    entropy_min: 3.5
+    confidence: high
+verify:
+  method: GET
+  url: https://huggingface.co/api/whoami-v2
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]