Simplify trim_entries, kill ContextBudget

trim_entries is now a simple loop:
1. Drop duplicate memories and DMN entries
2. While over budget: if memories > 50% of entry tokens, drop
   lowest-scored memory; otherwise drop oldest conversation entry
3. Snap to user message boundary

ContextBudget is gone — sections already have cached token totals:
- total_tokens() on ContextState replaces budget.total()
- format_budget() on ContextState replaces budget.format()
- trim() takes fixed_tokens: usize (system + identity + journal)

Co-Authored-By: Proof of Concept <poc@bcachefs.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

src/user/chat.rs

@@ -863,7 +863,7 @@ 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.context_budget().format();
+            app.status.context_budget = agent.context.format_budget();
             app.activity = agent.activities.last()
                 .map(|a| a.label.clone())
                 .unwrap_or_default();