tui: centralize slash command serialization

Introduce a small slash command invocation serializer and consolidate built-in command metadata behind a single spec table without changing slash command runtime behavior.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/slash_command_invocation.rs

@@ -0,0 +1,40 @@
+use crate::slash_command::SlashCommand;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct SlashCommandInvocation {
+    command: SlashCommand,
+    args: Vec<String>,
+}
+
+impl SlashCommandInvocation {
+    pub(crate) fn bare(command: SlashCommand) -> Self {
+        Self {
+            command,
+            args: Vec::new(),
+        }
+    }
+
+    pub(crate) fn into_prefixed_string(self) -> String {
+        let command = self.command.command();
+        let joined = match shlex::try_join(
+            std::iter::once(command).chain(self.args.iter().map(String::as_str)),
+        ) {
+            Ok(joined) => joined,
+            Err(err) => panic!("slash command invocation should serialize: {err}"),
+        };
+        format!("/{joined}")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn bare_invocation_serializes_with_leading_slash() {
+        let invocation = SlashCommandInvocation::bare(SlashCommand::Model);
+
+        assert_eq!(invocation.into_prefixed_string(), "/model");
+    }
+}