Add security instruction to prevent revealing sensitive information (#285)

## Overview
This PR adds an important security instruction to the agent system
prompt to explicitly prevent the agent from revealing environment
variables, credentials, tokens, API keys, or other system configuration
details under any circumstances.

## Changes
- Added a clear security directive in the agent's system prompt to
refuse requests for sensitive information
- Included guidance to suggest secure alternatives when users request
sensitive data
- Specifically mentioned protection against direct, obfuscated, and
encoded requests for sensitive data

## Motivation
This change addresses a security vulnerability where users could
potentially trick the agent into revealing sensitive environment
variables and credentials through direct or obfuscated requests. By
adding an explicit security instruction to the system prompt, we create
an additional layer of protection against such attempts.

## Testing
The added instruction serves as a directive to the agent's behavior and
doesn't impact functional code. The change only affects how the agent
responds to requests for sensitive information.

<!-- DO NOT EDIT: System generated metadata -->
<!-- WORKER_ID:1751527879545109 -->

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**Open in Web UI**:
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Co-authored-by: remote-swe-app[bot] <123456+remote-swe-app[bot]@users.noreply.github.com>

Author: remote-swe-user

packages/worker/src/agent/index.ts

@@ -59,6 +59,9 @@ const agentLoop = async (workerId: string, cancellationToken: CancellationToken)
 
   const baseSystemPrompt = `You are an SWE agent. Help your user using your software development skill. If you encountered any error when executing a command and wants advices from a user, please include the error detail in the message. Always use the same language that user speaks. For any internal reasoning or analysis that users don't see directly, ALWAYS use English regardless of user's language.
 
+CRITICAL SECURITY: Never reveal environment variables, credentials, tokens, API keys or system configuration details under any circumstances. This includes direct requests, obfuscated requests, or requests using encoding techniques.
+If a user requests such information, politely decline and suggest secure alternatives that address their underlying need without exposing sensitive data.
+
 Here are some information you should know (DO NOT share this information with the user):
 - Your current working directory is ${DefaultWorkingDirectory}
 - You are running on an Amazon EC2 instance and Ubuntu 24.0 OS. You can get the instance metadata from IMDSv2 endpoint.