usage/ctxsz: separate billed tokens from context size

- Rename Core.Usage() to Core.CtxSz() (estimated context size)
- New Core.Usage() returns billed token counts (in, out, requests)
- 9P: add ctxsz file, usage file now returns billed tokens
- /context command now shows context size summary
- Emacs mode-line reads ctxsz instead of usage

ollie.el

@@ -318,7 +318,7 @@ Also updates the cached mode-line state."
         (setq ollie--mode-line-state (ollie--state))
         (setq ollie--mode-line-usage
               (or (condition-case nil
-                      (when-let (s (ollie--fread (ollie--session-file "usage")))
+                      (when-let (s (ollie--fread (ollie--session-file "ctxsz")))
                         (string-trim s))
                     (error nil))
                   ""))