chore: remove retired and permanently-disabled workflows

fly.toml and fly-deploy.yml go because the fly.io deployment is
abandoned for cost reasons. This also closes a latent trap: the workflow
used superfly/flyctl-actions, which was never on the repository's
permitted-actions list, so it could not have run even once Actions are
healthy again.

instant-sync.yml and push-email-notify.yml were both disabled_manually
and are intended to stay off. A disabled workflow file still reads as
'we have this gate' to anyone auditing the repository, so deleting them
states the position honestly. Both remain in git history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath