chore(config): add JSON syntax validation to pre-push hook

Validates changed JSON files using python3 -m json.tool (zero
dependencies). Same diff-based scoping as existing checks — only
files changed vs remote are validated. Blocks push on syntax errors.

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

.config/hooks/pre-push

@@ -113,6 +113,31 @@ else
     echo "pre-push: skipping Python lint (ruff not found — install with: pip install 'ruff>=0.9')"
 fi
 
+# --- JSON syntax validation ---
+if git rev-parse --verify "$REMOTE_REF" >/dev/null 2>&1; then
+    JSON_FILES=$(git diff --name-only "$REMOTE_REF"..HEAD -- '*.json' 2>/dev/null || true)
+else
+    JSON_FILES=$(git ls-files '*.json')
+fi
+if [ -n "$JSON_FILES" ]; then
+    echo "pre-push: checking JSON syntax..."
+    JSON_FAIL=0
+    while IFS= read -r f; do
+        if [ -f "$REPO_ROOT/$f" ] && ! python3 -m json.tool "$REPO_ROOT/$f" >/dev/null 2>&1; then
+            echo "  FAIL: $f"
+            JSON_FAIL=$((JSON_FAIL + 1))
+        fi
+    done <<< "$JSON_FILES"
+    if [ "$JSON_FAIL" -gt 0 ]; then
+        echo "pre-push: FAIL — $JSON_FAIL JSON file(s) have syntax errors"
+        ERRORS=$((ERRORS + 1))
+    else
+        echo "pre-push: JSON — OK ($(echo "$JSON_FILES" | wc -l) file(s))"
+    fi
+else
+    echo "pre-push: no JSON changes — skipping"
+fi
+
 if [ "$ERRORS" -gt 0 ]; then
     echo ""
     echo "pre-push: $ERRORS check(s) failed. Push aborted."