feat(benchmark): add hook mode with baseline cache for pre-push checks

- Fix lint errors and LSP issues in benchmark.py
- Add --hook mode to compare against saved baseline
- Add --baseline flag to save results as baseline
- Add --threshold to configure degradation threshold (default 20%)
- Add benchmark step to pre-push hook in hk.pkl
- Update AGENTS.md with hk documentation links and benchmark runner docs

pyproject.toml

@@ -34,7 +34,7 @@ websocket = [
     "websockets>=12.0",
 ]
 sixel = [
-    "pysixel>=0.1.0",
+    "Pillow>=10.0.0",
 ]
 browser = [
     "playwright>=1.40.0",