proper Matrix threading + concise tool call formatting

agent_ux: uses Relation::Thread (not Reply) for tool call details.
format_tool_call extracts key params instead of dumping raw JSON.
format_tool_result truncated to 200 chars.

matrix_utils: added make_thread_reply() for threaded responses.
sync.rs routes ThreadReply engagement to threaded messages.

src/matrix_utils.rs

@@ -56,6 +56,19 @@ pub fn make_reply_content(body: &str, reply_to_event_id: OwnedEventId) -> RoomMe
     content
 }
 
+/// Build a threaded reply — shows up in Matrix threads UI.
+/// The thread root is the event being replied to (creates the thread on first use).
+pub fn make_thread_reply(
+    body: &str,
+    thread_root_id: OwnedEventId,
+) -> RoomMessageEventContent {
+    use ruma::events::relation::Thread;
+    let mut content = RoomMessageEventContent::text_markdown(body);
+    let thread = Thread::plain(thread_root_id.clone(), thread_root_id);
+    content.relates_to = Some(Relation::Thread(thread));
+    content
+}
+
 /// Send an emoji reaction to a message.
 pub async fn send_reaction(
     room: &Room,