Retire prepare-zones.sh pipeline; zones/ is now the served form

Big migration: the source/prepared split is gone. Each zones/*.zone is
now an RFC-compliant zone file that CoreDNS reads directly. Editing a
record is just edit + bump SOA + commit. CoreDNS auto-reloads within
30s; HE pulls on its own 300s SOA-refresh cycle.

Why: groundwork for the coredns-rfc2136 plugin to edit zones in place
without juggling a source/prepared transformation step. Also reduces
the mental model from "edit source, run prep, push" to just "edit".

Changes:
- zones/*.zone: 84 files migrated from Vultr-export form to RFC-compliant
  form (SOA injected, Vultr NS replaced with HE NS, CNAME/MX/NS rdata
  dot-terminated, apex lines get explicit @ prefix). Diff is mechanical
  and byte-count is unchanged (~340K) -- pure formatting promotion.
- docker-compose.yml: bind ./zones:/zones:ro (was ./zones-prepared)
- Makefile: dropped 'prep' target. 'reload' is now a no-op explainer.
  'tls-up' no longer depends on prep. 'clean' no longer wipes prepared.
- scripts/prepare-zones.sh moved to scripts/archive/ (kept for reference).
- .gitignore: updated comment for zones-prepared/ (now legacy).

NOT in this commit (follow-ups):
- CLAUDE.md updates documenting the new workflow.
- scripts/bump-serials.sh helper for manual-edit SOA bumping.
- coredns-rfc2136 plugin refactor (Phase 2b in the plan).

docker-compose.yml

@@ -53,7 +53,7 @@ services:
       - "${HEALTH_PORT}:8080/tcp"
     volumes:
       - ./Corefile:/etc/coredns/Corefile:ro
-      - ./zones-prepared:/zones:ro
+      - ./zones:/zones:ro
       # Subpath mount of Caddy's data dir. The healthcheck maintains
       # cert.pem / key.pem symlinks at the top of this tree, so CoreDNS
       # sees stable filenames regardless of hostname. The /accounts dir