fix: UTF-8-safe truncate (panic on Chinese mid-char)

Context shrink used &content[..200] which panics inside CJK codepoints.

crates/grokboy-core/src/agent.rs

@@ -124,6 +124,18 @@ fn tool_progress_line(name: &str, result_json: &str) -> String {
     format!("〔完成〕{name}")
 }
 
+/// Largest byte index ≤ `max` that sits on a UTF-8 char boundary.
+fn floor_char_boundary(s: &str, max: usize) -> usize {
+    if max >= s.len() {
+        return s.len();
+    }
+    let mut end = max;
+    while end > 0 && !s.is_char_boundary(end) {
+        end -= 1;
+    }
+    end
+}
+
 fn preview_progress(text: &str, max_chars: usize) -> String {
     let t = text.trim();
     if t.chars().count() <= max_chars {
@@ -217,7 +229,7 @@ pub fn truncate_messages(messages: &mut Vec<ChatMessage>, budget: usize) {
             for msg in messages.iter_mut() {
                 if let Some(content) = msg.content.as_mut() {
                     if content.len() > 200 {
-                        let keep = 200.min(content.len());
+                        let keep = floor_char_boundary(content, 200);
                         let omitted = content.len().saturating_sub(keep);
                         *content = format!("{}…\n[truncated {omitted} chars]", &content[..keep]);
                     }
@@ -617,6 +629,17 @@ mod tests {
     }
 
     #[test]
+    #[test]
+    fn floor_char_boundary_does_not_split_chinese() {
+        let s2 = "abcdefghij宣告";
+        // '告' is 3 bytes; index 11 lands inside it.
+        let idx = 11;
+        assert!(!s2.is_char_boundary(idx));
+        let end = floor_char_boundary(s2, idx);
+        assert!(s2.is_char_boundary(end));
+        let _ = &s2[..end];
+    }
+
     fn truncate_keeps_system_and_shrinks_old_tools() {
         let mut msgs = vec![
             ChatMessage::system("sys"),

crates/grokboy-core/src/tools.rs

@@ -382,15 +382,18 @@ async fn tool_write_file(ctx: &ToolContext, args: &Value) -> Result<Value> {
 
 fn truncate_output(s: &str, max: usize) -> String {
     if s.len() <= max {
-        s.to_string()
-    } else {
+        return s.to_string();
+    }
+    let mut end = max;
+    while end > 0 && !s.is_char_boundary(end) {
+        end -= 1;
+    }
     format!(
         "{}…\n[truncated {} bytes]",
-            &s[..max],
-            s.len().saturating_sub(max)
+        &s[..end],
+        s.len().saturating_sub(end)
     )
 }
-}
 
 #[cfg(test)]
 mod tests {