ALL-1003 (C): Relay HTTP/SSE consumer for Baton runtime

Relay-side half of the v0-banana path. Replaces the WS gateway client with a fetch+SSE consumer flag-gated on BATON_AGENT_HTTP_URL, mirrors the contract locked in #agent-ops 2026-05-21 (Authorization + X-Baton-Token, idempotencyKey UUID, BatonStreamEvent envelope, :keepalive comment frames). Keeps WS path as fallback for Slack-only soak + rollback. Branch talos/baton-http-sse-consumer. PR https://github.com/TextureHQ/mono/pull/10948.