[feat/pipeline-support] feat: add USE_PIPELINE environment variable for conditional pipeline feature activation

✨ Breaking Changes:
- Pipeline features are now opt-in via USE_PIPELINE environment variable

📝 Details:
- Pipeline 관련 도구들을 USE_PIPELINE 환경 변수로 제어 가능하도록 변경
- USE_GITLAB_WIKI, USE_MILESTONE과 동일한 패턴으로 구현
- 기본값은 false로 설정되어 pipeline 기능은 명시적으로 활성화해야 함
- README에 USE_PIPELINE 환경 변수 설명 추가

index.ts

@@ -192,6 +192,7 @@ const GITLAB_PERSONAL_ACCESS_TOKEN = process.env.GITLAB_PERSONAL_ACCESS_TOKEN;
 const GITLAB_READ_ONLY_MODE = process.env.GITLAB_READ_ONLY_MODE === "true";
 const USE_GITLAB_WIKI = process.env.USE_GITLAB_WIKI === "true";
 const USE_MILESTONE = process.env.USE_MILESTONE === "true";
+const USE_PIPELINE = process.env.USE_PIPELINE === "true";
 
 // Add proxy configuration
 const HTTP_PROXY = process.env.HTTP_PROXY;
@@ -614,6 +615,18 @@ const milestoneToolNames = [
   "get_milestone_burndown_events",
 ];
 
+// Define which tools are related to pipelines and can be toggled by USE_PIPELINE
+const pipelineToolNames = [
+  "list_pipelines",
+  "get_pipeline",
+  "list_pipeline_jobs",
+  "get_pipeline_job",
+  "get_pipeline_job_output",
+  "create_pipeline",
+  "retry_pipeline",
+  "cancel_pipeline",
+];
+
 /**
  * Smart URL handling for GitLab API
  *
@@ -2852,9 +2865,13 @@ server.setRequestHandler(ListToolsRequestSchema, async () => {
     ? tools0
     : tools0.filter(tool => !wikiToolNames.includes(tool.name));
   // Toggle milestone tools by USE_MILESTONE flag
-  let tools = USE_MILESTONE
+  const tools2 = USE_MILESTONE
     ? tools1
     : tools1.filter(tool => !milestoneToolNames.includes(tool.name));
+  // Toggle pipeline tools by USE_PIPELINE flag
+  let tools = USE_PIPELINE
+    ? tools2
+    : tools2.filter(tool => !pipelineToolNames.includes(tool.name));
 
   // <<< START: Gemini 호환성을 위해 $schema 제거 >>>
   tools = tools.map(tool => {