Add working-directory commands to the TUI (#38894)

## What changed

- Add `/cd [path]` for changing an idle local session's working directory while preserving conversation history. Relative paths resolve from the current directory, and omitting the path selects `~`.
- Reload project configuration, instructions, permissions, keybindings, file search, and hooks for the destination before replacing the active session.
- Reject unsafe transitions, including active or queued work, background terminals, remote environments, untrusted destinations, and incompatible permission profiles.
- Add `/pwd` and its `/cwd` alias to display the current working directory, and discard asynchronous results left over from the previous directory.

## Testing

- Cover command parsing, aliases, default paths, session eligibility, transition failures, history preservation, destination policy loading, hooks, and stale events.

GitOrigin-RevId: 1a987c029f157a087ba02c0f26e022260409132b

codex-rs/tui/src/bottom_pane/slash_commands.rs

@@ -309,6 +309,7 @@ mod tests {
                 SlashCommand::Diff,
                 SlashCommand::Mention,
                 SlashCommand::Status,
+                SlashCommand::Pwd,
                 SlashCommand::Usage,
             ]
         );

codex-rs/tui/src/bottom_pane/snapshots/codex_tui__bottom_pane__command_popup__tests__command_popup_default_items.snap

@@ -31,6 +31,8 @@ expression: commands
 /diff - show git diff (including untracked files)
 /mention - mention a file
 /status - show current session configuration and token usage
+/cd - change the current working directory
+/pwd - show the current working directory
 /title - configure which items appear in the terminal title
 /statusline - configure which items appear in the status line
 /theme - choose a syntax highlighting theme