Fix: Add missing return statement in run_prompt_filter function

The run_prompt_filter() function was missing a return statement after
filtering items. This caused it to return None, which would break the
--prompt_filter workflow when trying to write the filtered results to
the output file.

Changes:
- Added 'return filtered' at the end of run_prompt_filter() (line 253)

Impact:
- Fixes prompt_filter workflow when --prompt_filter flag is enabled
- Now correctly returns filtered items list instead of None

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
