fix: correct import for ControllerToAgentFileCallable (jenkins.agents package)

The class lives in jenkins.agents.ControllerToAgentFileCallable, not
jenkins.ControllerToAgentFileCallable. The wrong package caused a
NoClassDefFoundError at runtime, which silently prevented ForgeAIStep
from registering as a pipeline DSL step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: saurabh-oss

src/main/java/io/forgeai/jenkins/steps/DirectoryTreeCallable.java

@@ -1,7 +1,7 @@
 package io.forgeai.jenkins.steps;
 
 import hudson.remoting.VirtualChannel;
-import jenkins.ControllerToAgentFileCallable;
+import jenkins.agents.ControllerToAgentFileCallable;
 
 import java.io.File;
 import java.io.IOException;