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/api/mod.rs

@@ -651,10 +651,6 @@ pub async fn collect_stream(
                             let args: serde_json::Value =
                                 serde_json::from_str(&call.function.arguments).unwrap_or_default();
                             let args_summary = summarize_args(&call.function.name, &args);
-                            let _ = ui_tx.send(UiMessage::ToolCall {
-                                name: call.function.name.clone(),
-                                args_summary: args_summary.clone(),
-                            });
                             let is_background = args.get("run_in_background")
                                 .and_then(|v| v.as_bool())
                                 .unwrap_or(false);