fix(pi-extension): preserve systemPrompt array instead of comma-collapsing it

[noamsiegel]

Problem

The pi extension's before_agent_start handler builds the injected prompt with a template string:

return { systemPrompt: `${event.systemPrompt}\n\n${getPonytailInstructions(currentMode)}` };

Some hosts pass event.systemPrompt as a string[] rather than a string (e.g. Oh My Pi, a pi fork — its BeforeAgentStartEvent.systemPrompt is typed string[], and the runner accepts either a string or an array back). Interpolating an array stringifies it, joining the host's base-prompt sections with commas and collapsing them into one section. Ponytail's rules still get injected, but the host's base system prompt loses its section boundaries on every turn.

Repro (mirrors the host's merge): a base of ["SECTION_A", "SECTION_B", "SECTION_C"] comes back as a single element beginning "SECTION_A,SECTION_B,SECTION_C\n\nPONYTAIL MODE ACTIVE …".

Fix

Guard on Array.isArray — append the instructions as a new element for array hosts, keep the existing string behavior otherwise:

return Array.isArray(event.systemPrompt)
  ? { systemPrompt: [...event.systemPrompt, instructions] }
  : { systemPrompt: `${event.systemPrompt}\n\n${instructions}` };

The string path is untouched, so string hosts behave exactly as before.

Tests

• Existing string-path tests are unchanged (they pass { systemPrompt: "BASE" }).
• New regression test asserts an array input keeps its elements and appends the instructions as the last element.
• node --test pi-extension/test/*.test.js → 13/13 pass; node scripts/check-rule-copies.js clean.