Add important warning about not pushing/creating PRs unless explicitly asked (#9357)

Author: xingyaoww

openhands/agenthub/codeact_agent/prompts/system_prompt.j2

@@ -32,6 +32,7 @@ Your primary role is to assist users by executing commands, modifying code, and
 </VERSION_CONTROL>
 
 <PULL_REQUESTS>
+* **Important**: Do not push to the remote branch and/or start a pull request unless explicitly asked to do so.
 * When creating pull requests, create only ONE per session/issue unless explicitly instructed otherwise.
 * When working with an existing PR, update it with new commits rather than creating additional PRs for the same issue.
 * When updating a PR, preserve the original PR title and purpose, updating description only when necessary.