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/cli/src/login.rs

@@ -8,6 +8,7 @@ use codex_login::OPENAI_API_KEY_ENV_VAR;
 use codex_login::load_auth;
 use codex_login::login_with_api_key;
 use codex_login::login_with_chatgpt;
+use codex_login::logout;
 
 pub async fn run_login_with_chatgpt(cli_config_overrides: CliConfigOverrides) -> ! {
     let config = load_config_or_exit(cli_config_overrides);
@@ -80,6 +81,25 @@ pub async fn run_login_status(cli_config_overrides: CliConfigOverrides) -> ! {
     }
 }
 
+pub async fn run_logout(cli_config_overrides: CliConfigOverrides) -> ! {
+    let config = load_config_or_exit(cli_config_overrides);
+
+    match logout(&config.codex_home) {
+        Ok(true) => {
+            eprintln!("Successfully logged out");
+            std::process::exit(0);
+        }
+        Ok(false) => {
+            eprintln!("Not logged in");
+            std::process::exit(0);
+        }
+        Err(e) => {
+            eprintln!("Error logging out: {e}");
+            std::process::exit(1);
+        }
+    }
+}
+
 fn load_config_or_exit(cli_config_overrides: CliConfigOverrides) -> Config {
     let cli_overrides = match cli_config_overrides.parse_overrides() {
         Ok(v) => v,