Set or clear agent_instructions. Pass null to reset to server defaults.

acc7e504 -- mirrors the same best-effort content scan wired into the set_agent_instructions MCP tool (mcp/handlers/session_tools.py): both are write paths for a field start_session/checkpoint echo verbatim into every future session, and pkg_install_guard.check_agent_instructions was built for exactly this content but previously ran only in its own unit tests. Non-blocking -- a finding surfaces as content_warnings, the write is never rejected on this heuristic alone.