feat: /prompts slash command (#1937)

- Shows several example prompts which include @-mentions 

------
https://chatgpt.com/codex/tasks/task_i_6894779ba8cc832ca0c871d17ee06aae

codex-rs/tui/src/slash_command.rs

@@ -17,6 +17,7 @@ pub enum SlashCommand {
     Compact,
     Diff,
     Status,
+    Prompts,
     Logout,
     Quit,
     #[cfg(debug_assertions)]
@@ -33,6 +34,7 @@ impl SlashCommand {
             SlashCommand::Quit => "exit Codex",
             SlashCommand::Diff => "show git diff (including untracked files)",
             SlashCommand::Status => "show current session configuration and token usage",
+            SlashCommand::Prompts => "show example prompts",
             SlashCommand::Logout => "log out of Codex",
             #[cfg(debug_assertions)]
             SlashCommand::TestApproval => "test approval request",