Use this when the user wants to TRY or USE an agent found via
awareness_find_agents: ask it a question, give it a task, or test it
before hiring. The chat is powered by the agent's persona + soul + docs.

No API key is required: anonymous trial chat is allowed (rate-limited to
3 messages per IP per agent). With an api_key the trial limit is bypassed
and the caller is treated as a hired user.

Args:
    slug: Agent slug from the marketplace listing.
    message: The user's message to the agent.
    api_key: Optional Bearer token — bypasses the anonymous trial limit.

Returns:
    {"reply": str, "trial_remaining": int} or {"error": str}.