feat(subagent): add progress event handling and formatting for subagent execution

src/app/subagent/engine.rs

@@ -296,6 +296,17 @@ fn generate_workspace_tree(roots: &[std::path::PathBuf]) -> String {
     out
 }
 
+fn format_subagent_progress(prefix: &str, text: &str) -> String {
+    let lines: Vec<&str> = text.lines().filter(|l| !l.trim().is_empty()).collect();
+    if lines.is_empty() {
+        format!("{prefix}...")
+    } else if lines.len() == 1 {
+        format!("{prefix}: {}", lines[0])
+    } else {
+        lines[lines.len() - 2..].join("\n")
+    }
+}
+
 /// Synchronous subagent entry point: run up to `ctx.max_steps` iterations
 /// of the LLM tool loop.
 ///
@@ -369,6 +380,10 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
             anyhow::bail!("subagent aborted by parent at step {step}");
         }
 
+        let tx_clone = tx.clone();
+        let mut current_thinking = String::new();
+        let mut current_token = String::new();
+
         // Use streaming API so the abort flag is checked per SSE event,
         // making the subagent responsive to cancellation even during an
         // LLM call (non-streaming would block for 10-30s unchecked).
@@ -377,13 +392,24 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
             tdefs_opt.clone(),
             Some(0.7),
             Some(4096),
-            |_event| -> bool {
+            |event| -> bool {
                 // Check abort on every SSE event for responsive cancellation.
                 if ctx.abort_flag.as_ref().is_some_and(|f| f.load(std::sync::atomic::Ordering::SeqCst)) {
                     return false; // signals provider to abort
                 }
-                // We don't stream tokens to the UI for subagents — just
-                // need the assembled message at the end.
+                match event {
+                    crate::app::runtime::stream::StreamEvent::Reasoning(text) => {
+                        current_thinking.push_str(text);
+                        let prog = format_subagent_progress("thinking", &current_thinking);
+                        let _ = tx_clone.blocking_send(SubagentEvent::Progress(prog));
+                    }
+                    crate::app::runtime::stream::StreamEvent::Token(text) => {
+                        current_token.push_str(text);
+                        let prog = format_subagent_progress("replying", &current_token);
+                        let _ = tx_clone.blocking_send(SubagentEvent::Progress(prog));
+                    }
+                    _ => {}
+                }
                 true
             },
         );