chore: remove retired and permanently-disabled workflows (#333)

Housekeeping following the hosting decisions.

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

**`instant-sync.yml` + `push-email-notify.yml`** — both
`disabled_manually` and intended to stay off. A disabled workflow file
still reads as "we have this gate" to anyone auditing the repo, so
deleting them states the position honestly rather than leaving two dark
files behind.

All four remain in git history if any is ever wanted back.

Note CI cannot verify this PR — the repository-wide `startup_failure` is
still outstanding.

Author: hyperpolymath