feat(mcp): 协议层换成官方 Model Context Protocol SDK，删自研实现（裁决 10.4.6 + AGENTS §9.5）

- 新增依赖：github.com/modelcontextprotocol/go-sdk v1.8.0（direct）+ github.com/google/jsonschema-go v0.4.3
  （SDK 自带依赖，直接用于 schema 校验/默认值，避免再造一份）；go.sum 纯新增
- 删除自研协议层：rpc.go / stdio.go / http.go / sse.go 及其测试
- 用 SDK 的成熟逻辑：命令传输（stdio 子进程）/ SSE / Streamable HTTP、协议版本协商与
  -32022 回退、keepalive（含失败阈值）、tools/list_changed 订阅、能力协商（InitializeResult.Capabilities）
- 保留自研的四件事（其余零自研）：配置→传输、发现→registry 注册、能力族推断、schema 缓存（Prepare 命中不重复解析）
- 能力闸诚实化：服务器声明 capabilities 但不含 tools 时 ListTools/DiscoverTools 返回
  ErrCapabilityNotDeclared（不静默空集）；完全不声明 capabilities 的老式服务器仍按 legacy 处理
- 复核：build 绿 + capability 全包测试绿（mcp 42 用例，含真子进程 stdio / httptest Streamable HTTP /
  内存传输三类假 server，不依赖外网）+ `-race` 干净；变异 2 条（featuresFrom 恒 true / 删 applySchema 调用）已还原
- 无 REST/WS/env/数据目录变更；包内 API 有增删（见回报），仓外调用方签名未变（未改 cmd/cyrene 与 plugin/manager）

go.sum

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@@ -26,6 +32,8 @@ github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
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@@ -38,19 +46,29 @@ github.com/rogpeppe/go-internal v1.16.0 h1:O9DK+vNMDVGLr2BeZqmpLeMjiMNkuXfcqntWb
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