Add an in-session /resume command to Codex CLI

Problem

Codex CLI can resume saved sessions with codex resume SESSION_ID, but only after exiting the active TUI. Claude Code supports /resume from within an active session, so users can switch to saved work without leaving the interface.

This matters when someone starts a fresh session and then realizes they need to continue earlier work.

Requested behavior

• /resume opens the existing resume picker for the current workspace.
• /resume SESSION_ID switches directly to the specified saved session.
• The current session remains saved and can be resumed later.

Context

A similar request was raised in #4056, but it is closed and the feature is still unavailable in the current CLI.