refactor: move slash command handling into chatwidget (#2536)

no functional change, just moving the code that handles /foo into
chatwidget, since most commands just do things with chatwidget.

codex-rs/tui/src/app_event.rs

@@ -4,7 +4,6 @@ use ratatui::text::Line;
 
 use crate::history_cell::HistoryCell;
 
-use crate::slash_command::SlashCommand;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::SandboxPolicy;
 use codex_core::protocol_config_types::ReasoningEffort;
@@ -14,6 +13,9 @@ use codex_core::protocol_config_types::ReasoningEffort;
 pub(crate) enum AppEvent {
     CodexEvent(Event),
 
+    /// Start a new session.
+    NewSession,
+
     /// Request to exit the application gracefully.
     ExitRequest,
 
@@ -21,10 +23,6 @@ pub(crate) enum AppEvent {
     /// bubbling channels through layers of widgets.
     CodexOp(codex_core::protocol::Op),
 
-    /// Dispatch a recognized slash command from the UI (composer) to the app
-    /// layer so it can be handled centrally.
-    DispatchCommand(SlashCommand),
-
     /// Kick off an asynchronous file search for the given query (text after
     /// the `@`). Previous searches may be cancelled by the app layer so there
     /// is at most one in-flight search.