Docs: remove themed Santa executive summaries to keep root as canonical.\n\nWhy: Avoid duplication and ensure root exec summaries are the single source of truth.\nWhat: Delete _santa/EXECUTIVE_SUMMARY_SANTA_RALLY.md and _santa/EXECUTIVE_SUMMARY_SANTA_RALLY_CS.md.\nTested: ruff, black --check, mypy, pytest (32 passed).

Author: lieblm