pi-extension: event.systemPrompt missing returns literal 'undefined' prepended to prompt

In `pi-extension/index.js` lines 185-188: ``` return { systemPrompt: `${event.systemPrompt}\n\n${getPonytailInstructions(currentMode)}` }; ``` If `event` is a truthy object without a `systemPrompt` key (e.g., `{}`), template literal stringification produces the literal string `"undefined"` prepended to every system prompt: `"undefined\n\nPONYTAIL MODE ACTIVE..."`. The model sees this garbage prefix on every turn.