Kill UiMessage variants replaced by state-driven rendering

sync_from_agent reads directly from agent entries, so remove:
- UserInput (pending input shown from MindState.input)
- ToolCall, ToolResult (shown from entries on completion)
- ToolStarted, ToolFinished (replaced by shared active_tools)
- replay_session_to_ui (sync_from_agent handles replay)

-139 lines. Remaining variants are streaming (TextDelta, Reasoning),
status bar state, or ephemeral UI messages (Info, Debug).

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

src/agent/mod.rs

@@ -510,10 +510,6 @@ impl Agent {
 
         let args_summary = summarize_args(&call.function.name, &args);
         let _ = ui_tx.send(UiMessage::Activity(format!("calling: {}", call.function.name)));
-        let _ = ui_tx.send(UiMessage::ToolCall {
-            name: call.function.name.clone(),
-            args_summary: args_summary.clone(),
-        });
 
         // Spawn tool, track it
         let call_clone = call.clone();
@@ -561,10 +557,6 @@ impl Agent {
             ds.tool_errors += 1;
         }
 
-        let _ = ui_tx.send(UiMessage::ToolResult {
-            name: call.function.name.clone(),
-            result: output.clone(),
-        });
         self.active_tools.lock().unwrap().retain(|t| t.id != call.id);
 
         // Tag memory_render results for context deduplication