gog sheets notes can read cell notes from a range, but there's no way to write or clear them.

The Google Sheets API supports setting notes via batchUpdate with updateCells + fields: "note", so this should be straightforward to add.

Proposed usage

# Set a note on a cell
gog sheets update-note <id> "Sheet1!A1" --note "Hello world"

# Set from file (for multiline notes)
gog sheets update-note <id> "Sheet1!A1" --note-file notes.txt

# Clear a note
gog sheets update-note <id> "Sheet1!A1" --note ""

# Set same note on a range
gog sheets update-note <id> "Sheet1!A1:B2" --note "Applied to all 4 cells"

Naming follows the slides update-notes pattern. Could also alias as set-note.

I have a PR ready for this if you're interested.