Function send_message 

pub async fn send_message(
    store: &TaskStore,
    session_id: &str,
    channel: &str,
    channel_thread: Option<&str>,
    message: &str,
) -> Result<String>

High-level entry point: process a user message and return the assistant response.

Handles /model and /agent commands, assembles context, invokes the agent, parses the response for summaries, and stores both user and assistant messages.