feat: rewrite data-parity skill with interactive, plan-first workflow

Key changes based on feedback:
- Always generate TODO plan before any tool is called
- Enforce data_diff tool usage (never manual EXCEPT/JOIN SQL)
- Add PK discovery + explicit user confirmation step
- Profile pass is now mandatory before row-level diff
- Ask user before expensive row-level diff on large tables:
  - <100K rows: proceed automatically
  - 100K-10M rows: ask with where_clause option
  - >10M rows: offer window/partition/full choices
- Document partition modes (date/numeric/categorical) with examples
- Add warehouse_list as first step to confirm connections

Author: suryaiyer95