feat(01-02): embed loader, registry with Aho-Corasick, and filled test stubs

- loader.go with go:embed definitions/*.yaml for compile-time embedding
- registry.go with List(), Get(), Stats(), AC() methods
- Aho-Corasick automaton built from all provider keywords at NewRegistry()
- pkg/providers/definitions/ with 3 YAML files for embed
- All 5 provider tests pass: load, get, stats, AC, schema validation

pkg/providers/definitions/anthropic.yaml

@@ -0,0 +1,20 @@
+format_version: 1
+name: anthropic
+display_name: Anthropic
+tier: 1
+last_verified: "2026-04-04"
+keywords:
+  - "sk-ant-api03-"
+  - "anthropic"
+patterns:
+  - regex: 'sk-ant-api03-[A-Za-z0-9_\-]{93,}'
+    entropy_min: 3.5
+    confidence: high
+verify:
+  method: GET
+  url: https://api.anthropic.com/v1/models
+  headers:
+    x-api-key: "{KEY}"
+    anthropic-version: "2023-06-01"
+  valid_status: [200]
+  invalid_status: [401, 403]