Don't show deleted hunks when agent overwrites file (#29918)

Release Notes:

- Improved display of diffs when the agent rewrites a file from scratch.

crates/agent/src/thread.rs

@@ -898,7 +898,7 @@ impl Thread {
         if !loaded_context.referenced_buffers.is_empty() {
             self.action_log.update(cx, |log, cx| {
                 for buffer in loaded_context.referenced_buffers {
-                    log.track_buffer(buffer, cx);
+                    log.buffer_read(buffer, cx);
                 }
             });
         }