Implement +set-dropdown, +update-dropdown, +get-dropdown, and
+delete-dropdown shortcuts wrapping the v2 dataValidation API.
This resolves the issue where multipleValue writes silently
became plain text because the prerequisite dropdown configuration
step was not exposed as a CLI command.
Also add lark-sheets-formula.md reference for Lark-specific formula
rules (ARRAYFORMULA, native array functions, date diff, etc.) and
update the dropdown limitation note in SKILL.md to link to the new
+set-dropdown shortcut.
76a834e928