Add review delta tool to measure human review effort

New show-review-delta.sh compares AI-edited files (from impact log)
against git commits to show overlap percentage. High overlap means
most committed code was AI-generated with minimal human review.
Completes Phase 2 of the quantify-social-costs plan.

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

.claude/hooks/show-review-delta.sh

@@ -0,0 +1,123 @@
+#!/usr/bin/env bash
+#
+# show-review-delta.sh — Measure human review effort on AI-edited files.
+#
+# Compares the list of files the AI edited (from the impact log) against
+# recent git commits to estimate how much the user modified AI output
+# before committing.
+#
+# Usage: ./show-review-delta.sh [n_commits]
+#   n_commits: number of recent commits to analyze (default: 10)
+
+set -euo pipefail
+
+PROJECT_DIR="${CLAUDE_PROJECT_DIR:-$(cd "$(dirname "$0")/../.." && pwd)}"
+LOG_FILE="$PROJECT_DIR/.claude/impact/impact-log.jsonl"
+N_COMMITS="${1:-10}"
+
+if [ ! -f "$LOG_FILE" ]; then
+  echo "No impact log found at $LOG_FILE"
+  exit 0
+fi
+
+cd "$PROJECT_DIR"
+
+python3 -c "
+import json, sys, subprocess, os
+
+log_file = sys.argv[1]
+n_commits = int(sys.argv[2])
+project_dir = sys.argv[3]
+
+# Collect all AI-edited files across all sessions (latest entry per session)
+sessions = {}
+with open(log_file) as f:
+    for line in f:
+        try:
+            d = json.loads(line.strip())
+            sid = d.get('session_id', '')
+            if sid:
+                sessions[sid] = d
+        except Exception:
+            pass
+
+ai_files = {}  # file_path -> total edit count across sessions
+for entry in sessions.values():
+    for fp, count in entry.get('edited_files', {}).items():
+        # Normalize to relative path
+        rel = fp
+        if rel.startswith(project_dir + '/'):
+            rel = rel[len(project_dir) + 1:]
+        ai_files[rel] = ai_files.get(rel, 0) + count
+
+if not ai_files:
+    print('No AI-edited files found in impact log.')
+    print('(The edited_files field was added recently — older sessions lack it.)')
+    sys.exit(0)
+
+# Get recent commits and their changed files
+result = subprocess.run(
+    ['git', 'log', f'-{n_commits}', '--pretty=format:%H %s', '--name-only'],
+    capture_output=True, text=True, cwd=project_dir
+)
+
+commits = []
+current = None
+for line in result.stdout.split('\n'):
+    if not line.strip():
+        continue
+    if ' ' in line and len(line.split()[0]) == 40:
+        # Commit line
+        parts = line.split(' ', 1)
+        current = {'hash': parts[0], 'msg': parts[1], 'files': [], 'ai_files': []}
+        commits.append(current)
+    elif current is not None:
+        f = line.strip()
+        current['files'].append(f)
+        if f in ai_files:
+            current['ai_files'].append(f)
+
+print(f'=== Review Delta Analysis ({len(commits)} recent commits) ===')
+print(f'AI-edited files tracked: {len(ai_files)}')
+print()
+
+total_files = 0
+total_ai_overlap = 0
+
+for c in commits:
+    n_files = len(c['files'])
+    n_ai = len(c['ai_files'])
+    total_files += n_files
+    total_ai_overlap += n_ai
+
+    if n_ai > 0:
+        marker = ' ← AI-touched'
+    else:
+        marker = ''
+    print(f'  {c[\"hash\"][:8]} {c[\"msg\"][:60]}')
+    print(f'    {n_files} files changed, {n_ai} were AI-edited{marker}')
+    if n_ai > 0:
+        for f in c['ai_files']:
+            print(f'      {f} ({ai_files[f]} AI edits)')
+    print()
+
+if total_files > 0:
+    overlap_pct = total_ai_overlap * 100 / total_files
+else:
+    overlap_pct = 0
+
+print(f'=== Summary ===')
+print(f'  Total files in commits:     {total_files}')
+print(f'  Files also AI-edited:       {total_ai_overlap} ({overlap_pct:.0f}%)')
+print(f'  Files only human-edited:    {total_files - total_ai_overlap}')
+print()
+if overlap_pct > 80:
+    print('  High AI overlap — most committed code was AI-generated.')
+    print('  Consider reviewing more carefully to maintain skill.')
+elif overlap_pct > 40:
+    print('  Moderate AI overlap — mixed human/AI contribution.')
+elif total_ai_overlap > 0:
+    print('  Low AI overlap — mostly human-written code committed.')
+else:
+    print('  No AI overlap detected in recent commits.')
+" "$LOG_FILE" "$N_COMMITS" "$PROJECT_DIR"