fix(agent): 记录 PI runner 数据目录核对结果

启动后第一次核对通过时打一条不含路径的日志，便于确认 test 没有再打到 dev sidecar。

internal/agent/piagent.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log"
 	"net/http"
 	"os"
 	"os/exec"
@@ -191,6 +192,7 @@ func probeRunnerDataRoot(ctx context.Context, runnerURL string) (string, bool) {
 		return "", false
 	}
 	runnerDataRoots.Store(runnerURL, root)
+	log.Printf("pi runner data_dir check passed")
 	return root, true
 }
 
@@ -205,6 +207,7 @@ func workDirWithinRunnerRoot(workDir, root string) bool {
 func (p *PiAgentBackend) executeRemote(ctx context.Context, runnerURL string, req Request, workDir string, timeout time.Duration) (*Result, error) {
 	if root, ok := probeRunnerDataRoot(ctx, runnerURL); ok && !workDirWithinRunnerRoot(workDir, root) {
 		// Do not include either path: the caller classifies this phrase, and paths leak host layout.
+		log.Printf("pi runner data root mismatch; refusing /v1/run")
 		return &Result{Success: false, ErrorReason: "pi runner data root mismatch"}, nil
 	}