Cache context budget instead of recomputing every frame

budget() called tiktoken on every UI tick, which was the main CPU hog
during rapid key input. Move the cached ContextBudget onto ContextState
and recompute only when entries actually change (push_entry, compact,
restore_from_log).

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/chat.rs

@@ -828,7 +828,7 @@ impl ScreenView for InteractScreen {
             agent.expire_activities();
             app.status.prompt_tokens = agent.last_prompt_tokens();
             app.status.model = agent.model().to_string();
-            app.status.context_budget = agent.budget().status_string();
+            app.status.context_budget = agent.context.budget.status_string();
             app.activity = agent.activities.last()
                 .map(|a| a.label.clone())
                 .unwrap_or_default();