refactor: make user_id a required input, drop hardcoded jsahn default

user_id is now a required workflow_dispatch input (no default) and the
runner validates it is non-empty before doing anything. Removed the
'jsahn' default from the script arg and replaced jsahn with <user_id>
placeholders throughout TEST_WORKFLOW.md so the workflow is not pinned
to one account.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/run_agent_api.py

@@ -340,6 +340,8 @@ class AgentRunner:
 
     def run(self) -> int:
         try:
+            if not self.user_id:
+                raise SystemExit("ERROR: user_id 를 지정해야 합니다 (--user-id).")
             self.resolve_workspace()
             self.upload()
 
@@ -446,7 +448,7 @@ def parse_args() -> argparse.Namespace:
     p = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
     p.add_argument("--yaml-path", required=True, help="실행할 Agent YAML 경로 (repo 상대/절대)")
     p.add_argument("--api-base", default="https://legalpoc.eroomai.com/api")
-    p.add_argument("--user-id", default="jsahn")
+    p.add_argument("--user-id", default="", help="AgentBackend user_id (필수)")
     p.add_argument("--workspace-name", default="",
                    help="작업실 이름. GET /workspaces/lookup 으로 UUID 해석 (workspace-id 미지정 시)")
     p.add_argument("--workspace-id", default="",