Enable sandbox MCP gateway in dev.md workflow

- Add sandbox.mcp configuration with container and port
- Change workflow task from discussion poems to GitHub issues listing
- Update permissions to include issues:read
- Update toolsets from [discussions] to [issues]
- Add prompt to list the last 2 issues and verify the answer
- Remove safe-outputs configuration (not needed for issue listing)
- Compile dev.lock.yml with MCP gateway steps included

The dev.md workflow now tests the MCP gateway functionality by listing
recent GitHub issues and verifying the response.

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot