Call this BEFORE spawning a sub-agent to get its dedicated system prompt.
Use the returned activation_prompt as the sub-agent's system prompt so that
the main agent context stays clean and each sub-agent has an isolated, focused prompt.

Typical usage:
  1. awareness_init returns agent_profiles with slim summaries (agent_role, title, when_to_use).
  2. When a task matches a role, call awareness_get_agent_prompt(agent_role="<role>").
  3. Spawn a sub-agent with the returned activation_prompt as its system prompt.
  4. The sub-agent passes agent_role="<role>" to all awareness_* calls for memory isolation.

Args:
    agent_role: The agent role to fetch (e.g. "developer_agent", "reviewer_agent").
    memory_id: Target memory id (auto-read from X-Awareness-Memory-Id header if omitted).
    api_key: Bearer token for authentication.