Show context window usage per thread

[Daniel-Valembois]

Problem
There's currently no way to see how much of the context window a thread has used. You can set a context limit for Claude models, but you can't see how close a session is to it, so you only notice when quality drops or compaction kicks in.

Solution
A live usage indicator in the thread header or composer (for example 54k / 1M · 5%), updated after each turn. Codex shows this in its UI, and it's the one thing I miss after switching.

Additional context
The data should already be available, since the Claude harness records per-turn usage (input, cache-read and cache-creation tokens) in the session transcript. #14205 (custom context limits) and #14270 (context-window reporting for OpenCode) look related, so this may just be a matter of showing it in the UI.