Implemenent automatic difficulty level AI labeling for contribution opportunities

We already doing AI summary for issues, need to extend to to asses the complexity and label accordingly too.

No file, test, or entry point is named. Start by locating the existing AI issue-summary flow and determine how contribution opportunities are represented and labeled. Done should mean that complexity is assessed consistently and the resulting difficulty label is visible on opportunities, with tests covering the behavior.

Written by the indexing model from the issue text.