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/agent/context.rs

@@ -347,6 +347,44 @@ impl ConversationEntry {
         matches!(self, Self::Log(_))
     }
 
+    /// Short description for the debug UI.
+    pub fn label(&self) -> String {
+        let cfg = crate::config::get();
+        match self {
+            Self::System(_) => "system: [system prompt]".to_string(),
+            Self::Dmn(_) => "dmn: [heartbeat]".to_string(),
+            Self::Log(text) => {
+                let preview: String = text.chars().take(60).collect();
+                format!("log: {}", preview.replace('\n', " "))
+            }
+            Self::Memory { key, score, .. } => {
+                let role = "mem".to_string();
+                match score {
+                    Some(s) => format!("{}: [memory: {} score:{:.1}]", role, key, s),
+                    None => format!("{}: [memory: {}]", role, key),
+                }
+            }
+            Self::Message(m) => {
+                let role = match m.role {
+                    Role::Assistant => cfg.assistant_name.clone(),
+                    Role::User => cfg.user_name.clone(),
+                    Role::Tool => "tool".to_string(),
+                    Role::System => "system".to_string(),
+                };
+                if let Some(tc) = &m.tool_calls {
+                    let names: Vec<_> = tc.iter().map(|c| c.function.name.as_str()).collect();
+                    format!("{}: [tool_call: {}]", role, names.join(", "))
+                } else {
+                    let text = m.content_text();
+                    let preview: String = text.chars().take(60).collect();
+                    let preview = preview.replace('\n', " ");
+                    if text.len() > 60 { format!("{}: {}...", role, preview) }
+                    else { format!("{}: {}", role, preview) }
+                }
+            }
+        }
+    }
+
     /// Get a reference to the inner message.
     /// Panics on Log entries.
     pub fn message(&self) -> &Message {