Add runtime custom system prompt override

[Copilot]

Users can now override the default system prompt at runtime via Settings panel. Custom prompt persists in localStorage and takes precedence over default prompts/system_prompt.md, but MCP prompts still override both.

Changes

Frontend

• Added textarea in SettingsPanel for custom system prompt with clear button
• Pass custom_system_prompt through WebSocket to backend
• Added to DEFAULT_SETTINGS in useSettings hook

Backend

• Thread custom_system_prompt through WebSocket handler → ChatService → ChatOrchestrator → MessageBuilder
• MessageBuilder uses custom prompt when provided, falls back to default when empty/None

Tests

• Added 4 tests covering custom prompt usage, fallback behavior, and LLM integration

Priority Order

1. MCP prompts (selected_prompts)
2. Custom system prompt (Settings)
3. Default system prompt (file-based)

Example

# MessageBuilder now accepts optional custom_system_prompt
messages = await message_builder.build_messages(
    session=session,
    include_system_prompt=True,
    custom_system_prompt="You are a specialized coding assistant."  # Overrides default
)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: /usr/bin/curl curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>allow users to set a custom system prompt at run time.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes allow users to set a custom system prompt at run time. #153

---

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[github-actions]

🔒 Security Scan Results

Security Scan Summary

Scan Results

Python SAST (Bandit)

⚠️ Security issues found in Python code

Recommendations

• Review all SARIF files uploaded to GitHub Security tab
• Address high and critical severity vulnerabilities immediately
• Run npm audit fix and pip-audit locally to fix dependencies
• Consider implementing additional security controls

[github-actions]

🔒 Security Scan Results

Security Scan Summary

Scan Results

Python SAST (Bandit)

⚠️ Security issues found in Python code

Recommendations

• Review all SARIF files uploaded to GitHub Security tab
• Address high and critical severity vulnerabilities immediately
• Run npm audit fix and pip-audit locally to fix dependencies
• Consider implementing additional security controls