Function emit_session_start 

pub async fn emit_session_start(
    hook_bus: &Arc<HookBus>,
    session_id: &str,
) -> bool

Session-level: fire on_session_start for one session and store any inject as that session’s keyed injection (stream.rs reads it when composing the system prompt). Call AFTER discovery is known-finished — in-process hosts get that from spawn_discover_and_load itself; the daemon’s SessionActor awaits EngineHost::extensions_ready() first. Never re-runs discovery. Returns true when something was injected.