feat: add a /mention slash command (#2114)

- To help people discover @mentions.
- Command just places a @ in the composer.
- #2115 then improves the behavior of @mentions with empty queries.

codex-rs/tui/src/slash_command.rs

@@ -16,6 +16,7 @@ pub enum SlashCommand {
     Init,
     Compact,
     Diff,
+    Mention,
     Status,
     Prompts,
     Logout,
@@ -33,6 +34,7 @@ impl SlashCommand {
             SlashCommand::Compact => "summarize conversation to prevent hitting the context limit",
             SlashCommand::Quit => "exit Codex",
             SlashCommand::Diff => "show git diff (including untracked files)",
+            SlashCommand::Mention => "mention a file",
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Prompts => "show example prompts",
             SlashCommand::Logout => "log out of Codex",