[codex] Add slides text alignment visual checks

[florence-oai]

Summary

• Add visual-change-loop guidance for checking text alignment after Google Slides edits.
• Update the Google Slides visual iteration skill to inspect alignment-sensitive changes with thumbnails.

Validation

• Checked branch diff against origin/main.
• Confirmed PR [google-drive] Require local PNG inspection for URL thumbnails #148 and Tighten Google Drive file lifecycle skill #149 content is already present in the target base; their head commits cherry-picked cleanly as empty.