step-4: add revise.py, step4-review skill, README update

- revise.py: automatic pre-processing (ALL-CAPS→##, numbered sections→###,
  TOC removal, broken paragraph merging, whitespace normalization);
  supports N. and Na. numbering patterns; universal heuristics
- .claude/commands/step4-review.md: Claude Code skill for qualitative
  review of clean.md (🔴/🟡/🟢 report + interactive fixes)
- README: document step-4 workflow with revise.py and /step4-review
- .gitignore: exclude step-4/*/ and step-4/revision_log.md

.gitignore

@@ -33,3 +33,7 @@ step-2/*/
 # Output step-3 — profilo struttura generato da detect_structure.py
 step-3/*/
 
+# Output step-4 — MD revisionato e log generati da revise.py
+step-4/*/
+step-4/revision_log.md
+