fix: use static permissions in update-demos workflow (#2805)

GitHub Actions does not support expressions in the permissions block.
The dynamic expression was causing every workflow run to fail with
"workflow file issue" before any job could start.

Use static contents:write (matching pages.yaml convention). The deploy
step is already gated by `if: github.event_name == 'push'`.

Co-authored-by: Claude <noreply@anthropic.com>