feat: improve failure reporting with commit summaries

- Change end message from "failed to build" to "failed compilation"
- List failed commits with short hash (7 chars) and commit message
- Distinguish between sequential (stopped early) and parallel modes
- Makes it easier to identify which commits need fixing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

check-history.sh

@@ -372,7 +372,25 @@ fi
 # Final output
 # -----------------------------
 if [ "$FAIL" -eq 1 ]; then
-    echo "❌ Stopped early due to failure."
+    if [ "$JOBS" -eq 1 ]; then
+        echo "❌ Build failed - stopped at first failure."
+    else
+        echo "❌ Some commits failed compilation:"
+        echo
+        # List failed commits with their messages
+        for i in "${!COMMITS[@]}"; do
+            commit="${COMMITS[$i]}"
+            log="$LOGS_DIR/$commit.log"
+            if [ -f "$log" ] && grep -q "❌" "$log"; then
+                # Get first 7 characters of commit hash
+                short_hash="${commit:0:7}"
+                # Get commit message
+                commit_msg=$(git log -1 --format='%s' "$commit" 2>/dev/null || echo "(no message)")
+                echo "  $short_hash - $commit_msg"
+            fi
+        done
+        echo
+    fi
     debug "Cleaning up: removing $TMP_BASE"
 else
     echo "🎉 All commits passed!"