Phase 5 Batch 4: analyzer 规则引擎 + 3 个高层 Tool

- internal/analyzer: 纯函数规则引擎
  - Finding / Severity / Thresholds / Input / StageMetric / ExecutorMetric
  - 3 规则: data_skew (max/min ratio), gc_pressure (GC/CPU ratio),
    bottleneck (shuffle read+write GB)
  - 严重度阶梯: warning / critical (阈值 ×2)
  - Analyze 入口数据驱动 for ... range
  - 11 个单测 (3 规则 × normal/warning/critical + 零除防御)
- fetch_spark_metrics: SHS 业务封装, format=summary 触发 analyzer
- fetch_cluster_env: RM /cluster/info + /cluster/metrics 聚合
- analyze_spark_log: RM 日志降级链 + LLM-Ready Prompt 拼装
  - prompt 结构: cluster 元信息 + log source + log tail +
    heuristic findings + suggested LLM analysis
  - findings 是 first-pass filter, LLM 做最终判断
- deps.go: +AnalyzerThresholds
- main.go: 注入 cfg 的 3 个阈值
- 端到端实测: analyze_spark_log 完整 prompt 渲染

完成 11 个 Tool 表面 (list_clusters / spark_submit / 4 RM /
fetch_spark_metrics / fetch_cluster_env / analyze_spark_log /
fetch_url / upload_file)

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

internal/mcp/tools/fetch_cluster_env.go

@@ -0,0 +1,66 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"time"
+
+	"github.com/mark3labs/mcp-go/mcp"
+)
+
+const FetchClusterEnvName = "fetch_cluster_env"
+
+// NewFetchClusterEnvTool returns the schema for the fetch_cluster_env MCP Tool.
+func NewFetchClusterEnvTool() mcp.Tool {
+	return mcp.NewTool(FetchClusterEnvName,
+		mcp.WithDescription("Fetch YARN cluster environment information and metrics from the ResourceManager."),
+		mcp.WithString("cluster_id",
+			mcp.Required(),
+			mcp.Description("ID of the configured cluster (from list_clusters)"),
+		),
+	)
+}
+
+// FetchClusterEnvHandler fetches RM cluster info and metrics.
+func (d *Deps) FetchClusterEnvHandler(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+	clusterID, err := req.RequireString("cluster_id")
+	if err != nil {
+		return errResult("fetch_cluster_env: " + err.Error()), nil
+	}
+
+	callLog := startToolCall(ctx, d.Logger, FetchClusterEnvName, map[string]any{
+		"cluster_id": clusterID,
+	})
+	defer callLog.End()
+
+	cl, err := d.ClusterRepo.Get(ctx, clusterID)
+	if err != nil {
+		callLog.WithError(err)
+		return errResult("fetch_cluster_env: cluster " + clusterID + ": " + err.Error()), nil
+	}
+
+	base := strings.TrimRight(cl.RMURL, "/")
+	infoBody, err := d.fetchInternal(ctx, cl, base+"/ws/v1/cluster/info")
+	if err != nil {
+		callLog.WithError(err)
+		return errResult("fetch_cluster_env: info: " + err.Error()), nil
+	}
+	metricsBody, err := d.fetchInternal(ctx, cl, base+"/ws/v1/cluster/metrics")
+	if err != nil {
+		callLog.WithError(err)
+		return errResult("fetch_cluster_env: metrics: " + err.Error()), nil
+	}
+
+	var info, metrics any
+	_ = json.Unmarshal(infoBody, &info)
+	_ = json.Unmarshal(metricsBody, &metrics)
+
+	result := map[string]any{
+		"cluster_info": info,
+		"metrics":      metrics,
+		"fetched_at":   time.Now().UTC().Format(time.RFC3339),
+	}
+	callLog.WithResult(map[string]any{"info_bytes": len(infoBody), "metrics_bytes": len(metricsBody)})
+	return textResult(encodeJSON(result)), nil
+}