fix(cli): add --overwrite to drive download and slides thumbnail

[WadydX]

Summary

• require explicit --overwrite before replacing local files
• cover Drive downloads, Docs/Sheets/Slides exports, Docs tab exports, and Slides thumbnails
• preserve stdout output behavior
• add focused no-clobber and opt-in overwrite regression tests
• regenerate command docs

Closes #827

Verification

• focused command tests for every distinct writer path
• make ci
• live Google API proof for all six command paths: existing sentinel files were preserved without --overwrite; explicit overwrite produced valid PDF, XLSX, PPTX, and PNG artifacts
• autoreview local changes: clean
• autoreview complete PR branch: clean

[steipete]

Landed as fa3fa163.

Maintainer verification:

• Focused regression tests: go test ./internal/cmd -run 'TestDownloadDriveFile|TestRunDocsTabExport|TestDocsExportCmd_TabRouting|TestSlidesThumbnail|TestExecute_DocsExport_JSON' -count=1
• Full local gate: make ci
• Autoreview: local diff and full branch review both returned no actionable findings.
• Live Google API proof covered all six output paths: Drive download, Docs export, Docs tab export, Sheets export, Slides export, and Slides thumbnail. Each command refused an existing sentinel file without --overwrite, preserved its contents, then replaced it successfully with --overwrite and produced a valid artifact (PPTX, PDF, XLSX, or 1600x900 PNG as applicable).
• GitHub checks: Linux, Windows, macOS, worker, image, and Socket checks all passed.

Thanks @WadydX.