feat: add leo custom tier for personalized tool selection

- Add 'leo' tier as standalone (non-cumulative) tier
- Configure leo tier with curated tools for gmail, drive, docs, slides
- Updated tool_tier_loader to support standalone tiers
- Removed deprecated tools from leo tier (format_text_style, create_list, remove_list_formatting)

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

main.py

@@ -109,7 +109,7 @@ def main():
     )
     parser.add_argument(
         "--tool-tier",
-        choices=["core", "extended", "complete"],
+        choices=["core", "extended", "complete", "leo"],
         help="Load tools based on tier level. Can be combined with --tools to filter services.",
     )
     parser.add_argument(