[Bug] Conductor prompt injection surface in room-store initConductor

Summary

packages/core/src/stores/room-store.ts — initConductor() concatenates agentCatalog (from gateway) and userMessage (from user input) directly into the conductor system prompt without any escaping, validation, or length limits.

let prompt = buildConductorPrompt(agentCatalog);
if (userMessage) {
  prompt += `\n\n---\nUser task:\n${userMessage}`;
}

Expected behavior

• userMessage should be truncated to a reasonable max length
• agentCatalog should be validated for expected structure before embedding
• Consider wrapping user content in a fenced block to reduce injection surface

Files

• packages/core/src/stores/room-store.ts — initConductor() (line ~97)
• packages/shared/src/constants.ts — buildConductorPrompt()

Context

Introduced in PR #210.