acp: Add support for slash commands (#37304)

Depends on
https://github.com/zed-industries/agent-client-protocol/pull/45

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent2/src/agent.rs

@@ -292,6 +292,7 @@ impl NativeAgent {
                 action_log.clone(),
                 session_id.clone(),
                 prompt_capabilities_rx,
+                vec![],
                 cx,
             )
         });