Summary

Adds 7 new sheets subcommands covering formatting and layout
operations:

• merge / unmerge — merge and unmerge cell ranges (supports
  MERGE_ALL, MERGE_COLUMNS, MERGE_ROWS)
• number-format — apply number format types (NUMBER,
  CURRENCY, PERCENT, DATE, TIME, DATE_TIME, SCIENTIFIC, TEXT) with
  optional custom patterns
• freeze — freeze rows and/or columns, with --sheet
  selector defaulting to the first sheet
• resize-columns / resize-rows — set fixed pixel size or
  auto-fit to content (both support --auto)
• conditional-format — add boolean conditional formatting
  rules (NUMBER_GREATER, TEXT_CONTAINS, BLANK, etc.) with
  --bg-color, --text-color, and --bold styling

Also includes a fix for the build-breaking type grouping bug in
contacts_crud.go (duplicates #287, needed to compile).

Details

• All commands use BatchUpdate with the appropriate request
  types (MergeCells, RepeatCell, UpdateSheetProperties,
  UpdateDimensionProperties, AutoResizeDimensions,
  AddConditionalFormatRule)
• Follows existing patterns from sheets_format.go: input
  validation, normalizeGoogleID, cleanRange, parseSheetRange

• gridRangeFromMap for range resolution, dryRunExit, JSON
  output via outfmt

• resolveSheetIDByNameOrFirst is a new shared helper used by
  freeze and resize commands for sheet resolution without
  requiring a full A1 range
• parseSheetsColor supports both #RRGGBB and r,g,b (0-255
  integer) inputs
• parseColumnsSpan / parseRowsSpan handle dimension-only A1
  ranges (e.g. Sheet1!A:C, Sheet1!1:10)
• Freeze uses ForceSendFields to ensure zero values (unfreeze)
  are serialized correctly

Test plan

• make test passes (5 new test files with httptest mock
  servers)
• make fmt clean (gofumpt + goimports)
• make lint clean for all new files (remaining warnings
  are pre-existing in contacts/sheets_insert)
• Manual testing against a real spreadsheet (all 7 commands
  verified)

New files

• sheets_merge.go — merge/unmerge commands
• sheets_number_format.go — number-format command
• sheets_freeze.go — freeze command, shared sheet resolver helper
• sheets_resize.go — resize-columns/resize-rows, dimension span parsing
• sheets_conditional_format.go — conditional-format, color parsing, rule validation
• Five matching test files