Support the `/compact` command

The TypeScript CLI has this nifty /compact feature (introduced in https://github.com/openai/codex/pull/289) to summarize a conversation and use it as the starting point for a new conversation in order to "reclaim" context window. Ideally, model providers would implement this themselves, but as Codex supports third-party model providers, we cannot guarantee such a feature will always be available, so /compact is a nice workaround to have.