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/slash_command.rs

@@ -17,6 +17,7 @@ pub enum SlashCommand {
     Compact,
     Diff,
     Status,
+    Logout,
     Quit,
     #[cfg(debug_assertions)]
     TestApproval,
@@ -32,6 +33,7 @@ impl SlashCommand {
             SlashCommand::Quit => "Exit the application",
             SlashCommand::Diff => "Show git diff (including untracked files)",
             SlashCommand::Status => "Show current session configuration and token usage",
+            SlashCommand::Logout => "Log out of Codex",
             #[cfg(debug_assertions)]
             SlashCommand::TestApproval => "Test approval request",
         }