Fix four bugs from community PRs

- cache: skip directory source_file in save_cached to prevent IsADirectoryError (#444)
- report: skip structural-only communities with no real nodes (#443)
- hooks: allow @ in python path allowlist for Homebrew paths (#474)
- watch: keep source_file paths project-relative after rebuild (#434)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>