Demo GIF for shared-workbook + realtime

[shreydevkar]

The README hero GIF shows the single-user experience. Add a second GIF (or split-screen) showing two browser windows editing the same workbook with live cursors — that's the most undersold capability of the current build.

Acceptance criteria:

• GIF of two GridOS tabs editing the same workbook, with cursors visible on each side.
• README references it in the "Capabilities" section under Shared workbooks.