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/openai.yaml

@@ -0,0 +1,19 @@
+format_version: 1
+name: openai
+display_name: OpenAI
+tier: 1
+last_verified: "2026-04-04"
+keywords:
+  - "sk-proj-"
+  - "openai"
+patterns:
+  - regex: 'sk-proj-[A-Za-z0-9_\-]{48,}'
+    entropy_min: 3.5
+    confidence: high
+verify:
+  method: GET
+  url: https://api.openai.com/v1/models
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]