I've been rolling my own version of that recently via my https://github.com/simonw/llm-fragments-github plugin.

Here's an example where I condensed 50+ comments (accumulated over a year and a half!) into a summary comment and added it to the thread: https://github.com/simonw/sqlite-chronicle/issues/7#issuecom...

I created the summary like this:

  llm \
  -f issue:https://github.com/simonw/sqlite-chronicle/issues/7 \
  -m openai/o4-mini \
  -o reasoning_effort high \
  -s 'Output markdown explaining this issue and the proposed resolution succinctly and clearly'