run_ephemeral:
- Add briefing param (passed from spawn_agent, injected into system prompt)
- Subagent system prompt is now completely separate from parent's system_prompt:
    1. profile.subagent_system_prompt  (executor persona)
    2. custom_system_prompt            (role specialisation for this task)
    3. briefing                        (task context as system-level instruction)
  Fallback to profile.system_prompt only if subagent_system_prompt is not defined

spawn_agent:
- task → user message only (the goal)
- briefing → system prompt (credentials, context, instructions)
- system_prompt → role specialisation injected alongside briefing
- Removed old user-message composition (## Context / ## Task split)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>