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.mod

@@ -4,6 +4,7 @@ go 1.25.5
 
 require (
 	github.com/gin-gonic/gin v1.12.0
+	github.com/mark3labs/mcp-go v0.56.0
 	modernc.org/sqlite v1.53.0
 )
 
@@ -20,6 +21,7 @@ require (
 	github.com/go-playground/validator/v10 v10.30.1 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
+	github.com/google/jsonschema-go v0.4.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
@@ -32,8 +34,11 @@ require (
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.59.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
+	github.com/spf13/cast v1.7.1 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.1 // indirect
+	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/crypto v0.48.0 // indirect