Introduce a new /workflow command (#15854)

This subsumes the previous built-in prompt.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -30,6 +30,7 @@ pub mod search_command;
 pub mod symbols_command;
 pub mod tabs_command;
 pub mod term_command;
+pub mod workflow_command;
 
 pub(crate) struct SlashCommandCompletionProvider {
     cancel_flag: Mutex<Arc<AtomicBool>>,