docs: 新增「揭秘：隐藏功能与内部机制」文档栏目

添加 5 篇文档揭示 Claude Code 的三层功能门禁系统：
- 构建时 88+ feature flags 分类全解
- GrowthBook 运行时 A/B 测试体系与 tengu_* 命名文化
- KAIROS/PROACTIVE/BRIDGE 等 8 大未公开功能深度分析
- Ant 身份门控下的专属工具、命令与 Beta API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mint.json

@@ -76,6 +76,16 @@
         "docs/extensibility/skills",
         "docs/extensibility/custom-agents"
       ]
+    },
+    {
+      "group": "揭秘：隐藏功能与内部机制",
+      "pages": [
+        "docs/internals/three-tier-gating",
+        "docs/internals/feature-flags",
+        "docs/internals/growthbook-ab-testing",
+        "docs/internals/hidden-features",
+        "docs/internals/ant-only-world"
+      ]
     }
   ],
   "footerSocials": {