Kill publish_context_state() — screens lock the agent directly

F1 and F2 screens now call agent.context_state_summary() directly
via try_lock/lock instead of reading from a shared RwLock cache.
Removes SharedContextState, publish_context_state(), and
publish_context_state_with_scores().

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

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();
-            let sections = agent.shared_context.read().map(|s| s.clone()).unwrap_or_default();
+            let sections = agent.context_state_summary(None);
             app.status.context_budget = crate::agent::context::sections_budget_string(&sections);
             app.activity = agent.activities.last()
                 .map(|a| a.label.clone())