feat(registry): añade FileSystemPolicyStore con carga YAML versionada

Incluye 'yaml' en los overrides de mypy (PyYAML no trae type stubs).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/fixtures/policies/test_policy/index.yaml

@@ -0,0 +1,8 @@
+name: test_policy
+versions:
+  - id: v1
+    hash: deadbeef
+    author: pytest
+    message: fixture inicial
+    created_at: 2026-01-01T00:00:00Z
+active_version: v1

tests/fixtures/policies/test_policy/versions/v1.yaml

@@ -0,0 +1,13 @@
+name: test_policy
+version: v1
+description: Política de prueba
+input_validators:
+  - type: detect_pii
+    config:
+      entities: [PERSON, EMAIL]
+      severity_on_match: block
+output_validators:
+  - type: schema_match
+    config:
+      severity_on_mismatch: block
+on_validator_error: fail_closed