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>

.gitea/workflows/run-agent.yml

@@ -46,9 +46,8 @@ on:
         required: true
         type: string
       user_id:
-        description: 'AgentBackend user_id'
-        required: false
-        default: 'jsahn'
+        description: 'AgentBackend user_id (필수)'
+        required: true
         type: string
       workspace_name:
         description: '작업실 이름 (예: 팬아웃 테스트). GET /workspaces/lookup 으로 UUID 자동 해석'