feat: 完善 entries/ 格式 — 补全结构化字段、更新 README 和 Schema

- 所有 20 个 manifest.json 补全旧格式中的结构化字段
  - docker-app: category, fullDesc, stars, githubUrl, install
  - mcp: capabilities, toolCount, install, connection
  - http-api: endpoint, capabilities
- README 全面重写：entries/ 目录结构、manifest 字段说明、三种类型的添加示例
- SCHEMA_VERSION 升级 2.0.0 → 3.0.0
- 根 manifest.json 简化为 entries 统一统计

entries/db-mcp/manifest.json

@@ -15,5 +15,30 @@
     "macos",
     "windows",
     "linux"
-  ]
+  ],
+  "capabilities": [
+    "sql_query",
+    "schema_browse",
+    "data_export"
+  ],
+  "toolCount": 8,
+  "install": {
+    "method": "npx",
+    "packageName": "@modelcontextprotocol/server-postgres",
+    "command": "npx",
+    "args": [
+      "-y",
+      "@modelcontextprotocol/server-postgres",
+      "postgresql://localhost/mydb"
+    ]
+  },
+  "connection": {
+    "transport": "stdio",
+    "command": "npx",
+    "args": [
+      "-y",
+      "@modelcontextprotocol/server-postgres",
+      "postgresql://localhost/mydb"
+    ]
+  }
 }