Add logout command to CLI and TUI (#1932)

## Summary
- support `codex logout` via new subcommand and helper that removes the
stored `auth.json`
- expose a `logout` function in `codex-login` and test it
- add `/logout` slash command in the TUI; command list is filtered when
not logged in and the handler deletes `auth.json` then exits

## Testing
- `just fix` *(fails: failed to get `diffy` from crates.io)*
- `cargo test --all-features` *(fails: failed to get `diffy` from
crates.io)*

------
https://chatgpt.com/codex/tasks/task_i_68945c3facac832ca83d48499716fb51

codex-rs/tui/src/app.rs

@@ -328,6 +328,12 @@ impl App<'_> {
                     SlashCommand::Quit => {
                         break;
                     }
+                    SlashCommand::Logout => {
+                        if let Err(e) = codex_login::logout(&self.config.codex_home) {
+                            tracing::error!("failed to logout: {e}");
+                        }
+                        break;
+                    }
                     SlashCommand::Diff => {
                         let (is_git_repo, diff_text) = match get_git_diff() {
                             Ok(v) => v,