feat: 迁移统一目录元数据契约 (#102)

## 中文

- 为 System Agent、34 个 Builtin Skill、28 个 Pointer/Collection 条目增加 catalog
sidecar
- 覆盖 147 个 collection child，并固定可证明的来源；无法证明的内容保持 listing-only/unknown
- 增加 strict Schema、validator、collection check 与 CI 完整性门禁

## English

- Add catalog metadata sidecars for the System Agent, 34 built-in
Skills, and 28 pointer/collection entries
- Cover 147 collection children while keeping unverifiable facts
listing-only or unknown
- Add strict schemas, validators, deterministic collection checks, and
CI completeness gates

## 验证 / Verification

- Catalog validator 17/17
- Collection generator 4/4
- 63 sidecars, 147 children, zero errors

skills/follow-builders/entry.json

@@ -28,6 +28,7 @@
   "source": {
     "kind": "git",
     "repoUrl": "https://github.com/zarazhangrui/follow-builders.git",
-    "repoBranch": "main"
+    "repoBranch": "main",
+    "ref": "239227ddea46dfb9db6ed23e5864a5fabb9cee83"
   }
 }