fix: isolate golangci-lint cache per job to prevent parallel conflicts

Add GOLANGCI_LINT_CACHE and XDG_CACHE_HOME environment variables
pointing to job-specific cache directory to prevent parallel job
conflicts when running golangci-lint.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/app/run/runner.go

@@ -42,7 +42,6 @@ type Runner struct {
 	runningTasks sync.Map
 }
 
-
 // getJobCacheDir returns a job-isolated cache directory
 func (r *Runner) getJobCacheDir(taskID int64) string {
 	return filepath.Join(r.cfg.Host.WorkdirParent, "jobs", fmt.Sprintf("%d", taskID))
@@ -65,7 +64,7 @@ func (r *Runner) CleanStaleJobCaches(maxAge time.Duration) {
 	if err != nil {
 		return // directory may not exist yet
 	}
-	
+
 	cutoff := time.Now().Add(-maxAge)
 	for _, entry := range entries {
 		if !entry.IsDir() {