Feature Request: Add context window visibility

Q chat needs to add visibility into how much context window has been consumed, especially when the amount of remaining context is getting low. Claude Code has a nice UX for this to give you some ideas.

1. Doesn't need to be super precise/accurate. Just a rough % is fine. Knowing that it's 24.32% vs 26.81% is not important to me. I'm looking for whether it's ~20% vs ~60%.
2. With the new `/context` feature, it's important to know what % is being taken up by context files. For example, `/context show --expand` could show what % context files are taking up. Or if you go with Claude Code style where they show the amount somewhere near the prompt, split it into % taken up by context files and % taken up by chat.