feat(sheets): clear table data rows

[steipete]

Summary

• add gog sheets table clear for header-safe table data row clearing
• compute and expose table dataA1, skipping footer rows when Sheets reports one
• require --force for mutation while keeping --dry-run --json useful for range preview
• update README, Sheets table docs, generated command docs, and changelog

Verification

• go test ./internal/cmd -run 'TestSheetsTable' -count=1
• make build
• make lint
• make test
• make docs-site
• make ci
• Live Google smoke with clawdbot@gmail.com: created disposable spreadsheet 14Jn5sl4Umj5H9RyBDpYzw92P1jVzjnSzAiPm6meMlGA, created table 308533532, appended row at Sheet1!A5:C5, verified no-force clear fails, cleared Sheet1!A2:C5 with --force, verified header stayed, data row removed, table object remained, then trashed the spreadsheet.