`approval: done` not re-added after new approvals follow a push

[quarckster]

After a push dismisses reviews and the bot adds approval: review pending, when approvals reach threshold again, the bot fails to restore approval: done.

Reproduce:

1. PR reaches 2 committer approvals → bot adds approval: done
2. New commit pushed → reviews dismissed → bot removes approval: done, adds approval: review pending
3. PR gets 2 new committer approvals → bot should add approval: done but doesn't

Root cause: The webhook payload labels may be stale. When checking whether to add approval: done, the bot uses existing_labels from the event payload (which may still show the old state before removal), so it skips the add.