Subscribe to real-time briefing-render events. Returns the SSE endpoint URL with the chosen filters as query params — the agent's MCP client should open it with EventSource (browser), httpx.stream / aiohttp (Python), or `curl -N` (CLI). Event types: `briefing.rendered` (daily-brief lands), `declassified.published` (new Declassified episode), `persona_briefing.rendered` (persona brief synthesised / audio rendered). Frame shape: {event_type, seq, slug, vertical, persona_slug, audio_url, published_at, metadata}. The endpoint replays the last ~1000 events on connect; a heartbeat is emitted every 30s. Public-anon read.