Fix Lightning RLM child tool routing

[pst2154]

Summary

• Clarify that RLM children must invoke installed Python modules through the advertised ipython tool.
• Add an opt-in Nemotron 3.5 Lightning extension that sets temperature to zero and constrains child tool selection to ipython until a parent reply is delivered.
• Document the extension and cover the prompt contract with tests.

Why

Nemotron 3.5 Lightning can emit unadvertised calls such as agent_message instead of executing agent_message.send() inside IPython. Those calls fail before the child can return its findings. The extension provides a model-specific request guard without hardcoding the workaround into Prime Agent core.

Validation

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/system-prompt.test.ts — 26 tests passed.
• npm run check — passed.
• Prime RLM smoke test — the Lightning child selected ipython on its first response, delivered exactly one parent message, and stopped normally.

Scope

This fixes Lightning's RLM tool-routing failure. It does not claim to improve the model's underlying SWE reasoning accuracy.