Restore entry labels in context tree: role, tool calls, memory keys

ConversationEntry::label() provides descriptive labels matching the
old entry_sections format:
- "Kent: what about..." / "Aria: [tool_call: memory_search, ...]"
- "mem: [memory: key-name score:0.73]"
- "dmn: [heartbeat]" / "system: [system prompt]"

Uses config names (assistant_name, user_name) not generic "asst"/"user".
Widget renderer uses label() instead of raw content preview.

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

src/user/widgets.rs

@@ -220,18 +220,16 @@ impl SectionTree {
             for ce in section.entries() {
                 let entry_selected = self.selected == Some(*idx);
                 let entry_expanded = self.expanded.contains(idx);
-                let text = ce.entry.message().content_text();
-                let preview: String = text.chars().take(60).collect();
-                let preview = preview.replace('\n', " ");
-                let label = if preview.len() < text.len() {
-                    format!("    {}...", preview)
+                let text = if ce.entry.is_log() {
+                    String::new()
                 } else {
-                    format!("    {}", preview)
+                    ce.entry.message().content_text().to_string()
                 };
-                let entry_marker = if text.len() > 60 {
+                let has_content = text.len() > 0;
+                let entry_marker = if has_content {
                     if entry_expanded { "▼" } else { "▶" }
                 } else { " " };
-                let entry_label = format!("  {} {:>6} {}", entry_marker, ce.tokens, label);
+                let entry_label = format!("  {} {:>6} {}", entry_marker, ce.tokens, ce.entry.label());
                 let style = if entry_selected {
                     Style::default().fg(Color::Yellow).add_modifier(Modifier::BOLD)
                 } else {