add slash resume (#7302)

`codex resume` isn't that discoverable. Adding it to the slash commands
can help

codex-rs/tui/src/chatwidget.rs

@@ -1482,6 +1482,9 @@ impl ChatWidget {
             SlashCommand::New => {
                 self.app_event_tx.send(AppEvent::NewSession);
             }
+            SlashCommand::Resume => {
+                self.app_event_tx.send(AppEvent::OpenResumePicker);
+            }
             SlashCommand::Init => {
                 let init_target = self.config.cwd.join(DEFAULT_PROJECT_DOC_FILENAME);
                 if init_target.exists() {