Phase 5 Batch 1: executor + list_clusters + spark_submit + /mcp 路由

- internal/executor: spark-submit 进程执行, exec.Command 切片形式
  (绝不 shell) + stdout 10MiB tail-cap + app_id regex
  + 5 个单测覆盖 slice-form 注入防御 + 退出码
- internal/mcp/server.go: NewServer + Streamable HTTP Handler
- internal/mcp/tools/{list_clusters, spark_submit, deps, helpers}.go
- mcp-go API 实测修正: WithArray 用 Items() 声明 item 类型;
  StreamableHTTPServer 直接实现 http.Handler, 无 .Handler() 方法
- main.go: 挂 /mcp 路由 (AgentAuth + mcpHandler 手动组合)

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

go.sum

@@ -9,8 +9,12 @@ github.com/cloudwego/base64x v0.1.6/go.mod h1:OFcloc187FXDaYHvrNIjxSe8ncn0OOM8gE
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@@ -42,8 +48,14 @@ github.com/json-iterator/go v1.1.12 h1:PV8peI4a0ysnczrg+LtxykD8LfKY9ML6u2jnxaEnr
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