feat: add support for commands in the Rust TUI (#935)

Introduces support for slash commands like in the TypeScript CLI. We do
not support the full set of commands yet, but the core abstraction is
there now.

In particular, we have a `SlashCommand` enum and due to thoughtful use
of the [strum](https://crates.io/crates/strum) crate, it requires
minimal boilerplate to add a new command to the list.

The key new piece of UI is `CommandPopup`, though the keyboard events
are still handled by `ChatComposer`. The behavior is roughly as follows:

* if the first character in the composer is `/`, the command popup is
displayed (if you really want to send a message to Codex that starts
with a `/`, simply put a space before the `/`)
* while the popup is displayed, up/down can be used to change the
selection of the popup
* if there is a selection, hitting tab completes the command, but does
not send it
* if there is a selection, hitting enter sends the command
* if the prefix of the composer matches a command, the command will be
visible in the popup so the user can see the description (commands could
take arguments, so additional text may appear after the command name
itself)


https://github.com/user-attachments/assets/39c3e6ee-eeb7-4ef7-a911-466d8184975f

Incidentally, Codex wrote almost all the code for this PR!

codex-rs/tui/src/slash_command.rs

@@ -0,0 +1,36 @@
+use std::collections::HashMap;
+
+use strum::IntoEnumIterator;
+use strum_macros::AsRefStr; // derive macro
+use strum_macros::EnumIter;
+use strum_macros::EnumString;
+use strum_macros::IntoStaticStr;
+
+/// Commands that can be invoked by starting a message with a leading slash.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, EnumString, EnumIter, AsRefStr, IntoStaticStr)]
+#[strum(serialize_all = "kebab-case")]
+pub enum SlashCommand {
+    Clear,
+    Quit,
+}
+
+impl SlashCommand {
+    /// User-visible description shown in the popup.
+    pub fn description(self) -> &'static str {
+        match self {
+            SlashCommand::Clear => "Clear the chat history.",
+            SlashCommand::Quit => "Exit the application.",
+        }
+    }
+
+    /// Command string without the leading '/'. Provided for compatibility with
+    /// existing code that expects a method named `command()`.
+    pub fn command(self) -> &'static str {
+        self.into()
+    }
+}
+
+/// Return all built-in commands in a HashMap keyed by their command string.
+pub fn built_in_slash_commands() -> HashMap<&'static str, SlashCommand> {
+    SlashCommand::iter().map(|c| (c.command(), c)).collect()
+}