step-6: add fix_chunks.py, make step-6 self-contained

- verify_chunks.py now reads from step-6/<stem>/chunks.json and
  auto-copies from step-5 on first run (input and output both in step-6)
- fix_chunks.py: new script that applies fixes directly on chunks.json
  (merge too-short/incomplete, split too-long, remove empty, add prefix)
  supports --dry-run to preview changes before applying
- step6-fix.md skill updated to use fix_chunks.py workflow:
  dry-run → user approval → apply → re-verify

.gitignore

@@ -40,3 +40,6 @@ step-4/revision_log.md
 # Output step-5 — chunk generati da chunker.py
 step-5/*/
 
+# Output step-6 — report generati da verify_chunks.py
+step-6/*/
+