feat(dns): per-environment BASE_DOMAIN — prod moves to its own subdomain

INCIDENT FIX. Prod's external-dns DELETED EVERY ONE OF DEV'S PUBLIC DNS RECORDS. Both
clusters ran policy: sync against the SAME Cloudflare zone with the SAME implicit
txtOwnerId (`default`), so prod believed it owned dev's records, found no matching sources,
and removed them plus their TXT registry entries. Dev recovered ~30s after prod was scaled
to 0; verified restored against Cloudflare directly (my first check went through a pfSense
override and falsely showed healthy — local resolvers are not evidence about public DNS).

THE MISS WAS MINE, and it was a failure of imagination in how I briefed every agent. I put
"never touch dev" in brief after brief and enforced it — separate kubeconfigs, verified
--limit, sha-checked dev's kubeconfig before and after. Every one of those guards held. NONE
of them covered SHARED EXTERNAL RESOURCES. Prod reconciling its OWN correctly-authored
manifests reached across a third party and destroyed dev's state. Cluster-to-cluster
isolation was verified; the dependency that mattered was cluster-to-Cloudflare.

Operator's architecture: dev -> *.dev.home.keithjackson.dev, prod ->
*.prod.home.keithjackson.dev, apex CNAME <svc>.home -> <svc>.prod.home.

PHASE 1 ONLY (this commit): prod moves to its own subdomain. DEV IS NOT MIGRATED and its
effective hostname stays home.keithjackson.dev, so dev renders byte-identically and nothing
live moves. That alone ends the collision and unblocks prod. Phase 2 (dev -> dev.home. +
apex CNAMEs) is deferred to when the operator is present: it relocates Keycloak's issuer and
12 clients' redirect URIs, forces a rolling k3s restart and kubectl SSO re-issue for every
user, and moves gitea.home. — which is BOTH the GitOps source and the current push target.

BASE_DOMAIN WAS A DEAD KNOB — it existed in cluster-vars but the status report flagged it as
referenced by nothing, because hostnames were hardcoded throughout. Now real: 141 refs.
Searched the AUTHORITATIVE reconciled set by resolving each Flux spec.path transitively via
conformance/lib/status.py — the same resolver the L0 gate uses, so "what Flux reconciles"
has one definition. 185 occurrences across 47 files in 154 reconciled files. Tokenized:
Gateway listeners, the wildcard Certificate, coredns-custom, external-dns, Keycloak
instance/realm (26 redirect URIs), oauth2-proxy, Grafana OIDC, all HTTPRoutes, Homepage's 15
links, 30 runbook_urls, unifi cloud-init, render.py's ZONE constant (which drove
--oidc-issuer-url/--cookie-domain/--whitelist-domain on every forward-auth sidecar) and 11
catalog/ ServiceDefinitions re-rendered canonically. Left alone: 43 comment/prose
occurrences and non-reconciled paths.

TWO THAT CANNOT BE TOKENIZED, flagged not forced:
  - headlamp's OIDC URLs live inside a SOPS-encrypted Secret; re-encrypting rewrites the MAC
    over every value and the OFFLINE harness cannot verify it (kustomize does not decrypt),
    so an error would break live-dev SSO with no pre-flight proof. Prod needs its own copy
    anyway — client secrets are realm-scoped.
  - kube-apiserver --oidc-issuer-url is a k3s SERVER FLAG, not a manifest, so Flux can never
    reach it. Done on the inventory seam instead (k3s_base_domain -> k3s_oidc_issuer_url,
    overridden in group_vars/k3s_prod). Verified per host: dev keeps the default, prod gets
    prod.home.

EXTERNAL-DNS OWNER, the actual incident cause: txtOwnerId is now explicit per environment —
dev renders exactly `default` (the chart default it was already using, so its existing
records stay recognised) and prod gets `prod`. The BASE default is a NON-COLLIDING SENTINEL,
deliberately not `default`, so a future environment that forgets the knob owns nothing and
can delete nothing rather than silently inheriting dev's identity.

I OVERRODE ONE AGENT DECISION against our own prior finding: it narrowed prod's
domainFilters to prod.home.keithjackson.dev as defence in depth. FINDING F-DNS1
(DECISIONS-LOG 2026-08-17) already proved a subdomain filter DROPS THE PARENT ZONE —
ExternalDNS generates the record then skips it with "no hosted zone matching record DNS
Name". Prod would have published NOTHING. Set to the ZONE (keithjackson.dev) like dev.
Isolation comes from the subdomain split plus txtOwnerId, which is what the incident
actually needed and what a domain filter cannot provide.

DEV PROVEN UNCHANGED: 11 of 12 substituting paths byte-identical; the 12th differs by
exactly one line, `+ txtOwnerId: default` — the value dev was already using implicitly.
POSITIVE CONTROLS confirm the harness is not blind: perturbing BASE_DOMAIN moves 10 of 12
hashes (the 2 that don't provably contain no hostname), perturbing the owner id moves
exactly 1. Prod's rendered output has ZERO home.keithjackson.dev references outside prod.,
and its cert renders *.prod.home.keithjackson.dev — no overlap with dev's.

L0 substitution assertions 106 -> 254; BASE_DOMAIN is no longer flagged dead. Gate PASS.

PROD REMAINS SUSPENDED (platform + 10 downstream tiers) pending review; nothing applied.

Serves: R-ENV-02, R-DNS-01, R-DNS-02, R-DNS-03, R-TLS-01.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

clusters/base/cluster-vars.yaml

@@ -17,9 +17,63 @@ data:
   CLUSTER_ENV: "base"                       # dev | prod  (overlays set this)
 
   # --- DNS / TLS ---
-  BASE_DOMAIN: "home.keithjackson.dev"      # platform hostname suffix (same both envs; split-horizon)
-  DNS_ZONE: "keithjackson.dev"              # the actual Cloudflare ZONE (BASE_DOMAIN is a subdomain of it);
-                                            # ExternalDNS domain-filter must be the zone, not the subdomain
+  # THE platform hostname suffix. Every public hostname the cluster serves is `<svc>.${BASE_DOMAIN}`,
+  # the wildcard cert is `*.${BASE_DOMAIN}`, coredns-custom answers the `${BASE_DOMAIN}` zone with
+  # ${GATEWAY_VIP}, and every OIDC issuer/redirect URI is built from it.
+  #
+  # THIS KNOB WAS DEAD UNTIL 2026-09-04 — declared here but referenced by NO manifest, because every
+  # hostname was hardcoded. That is what made the 2026-09-04 incident possible: prod and dev were
+  # authored to serve the SAME hostnames out of the SAME Cloudflare zone, so prod's external-dns
+  # (policy: sync, owner `default`) concluded it owned dev's public records and DELETED ALL OF THEM.
+  # See infra/PROD-PROVISIONING.md §10. The fix is architectural: each environment gets its OWN
+  # subdomain, so the two clusters can no longer describe the same name.
+  #
+  #   dev   -> home.keithjackson.dev            (UNCHANGED — see below)
+  #   prod  -> prod.home.keithjackson.dev
+  #
+  # DEV DELIBERATELY STAYS ON THE APEX `home.keithjackson.dev`. The target architecture puts dev on
+  # `dev.home.keithjackson.dev` with apex CNAMEs `<svc>.home` -> `<svc>.prod.home`, but moving dev
+  # relocates its Keycloak issuer, every OIDC client redirect URI, kubectl SSO, its wildcard cert and
+  # the Gitea host that IS the GitOps source — on a live cluster. Deferred to phase 2
+  # (infra/PROD-PROVISIONING.md §10). Base default is therefore the DEV value, the same convention as
+  # GATEWAY_VIP / LONGHORN_DATA_PATH: the base default is the known-good working value, dev restates
+  # it verbatim, and prod overrides it. Dev's rendered output is byte-identical to the pre-
+  # tokenization hardcoded manifests (verified 2026-09-04, all 12 substituted paths).
+  BASE_DOMAIN: "home.keithjackson.dev"
+
+  # ExternalDNS `--domain-filter` (platform/externaldns/helmrelease.yaml). SAFETY NET #2 behind
+  # EXTERNALDNS_TXT_OWNER_ID: a record whose name falls outside this filter is not even considered,
+  # so it cannot be created, updated or deleted regardless of TXT ownership.
+  #
+  # DEV KEEPS THE ZONE APEX `keithjackson.dev`, and must: dev serves the `home.keithjackson.dev`
+  # apex itself, and the Cloudflare provider filters the ACCOUNT'S ZONES by this value — a filter
+  # that is a strict subdomain of the zone dropped the parent zone outright ("no hosted zone matching
+  # record DNS Name", DECISIONS-LOG 2026-08) on the version in use then. So dev's value is unchanged
+  # from the retired `DNS_ZONE` knob this replaces, and dev renders byte-identically.
+  #
+  # PROD NARROWS TO ITS OWN SUBTREE `prod.home.keithjackson.dev`. VERIFY ON PROD'S FIRST RECONCILE
+  # (infra/PROD-PROVISIONING.md §10): if the zone-lookup limitation above still applies to the pinned
+  # chart, prod's external-dns will select NO zone and create NO records. That failure is FAIL-CLOSED
+  # — it can never delete another environment's records — and the fallback is to set prod's value
+  # back to "keithjackson.dev", leaving EXTERNALDNS_TXT_OWNER_ID as the sole (and sufficient) guard.
+  EXTERNALDNS_DOMAIN_FILTER: "keithjackson.dev"
+
+  # ExternalDNS TXT registry owner id (`--txt-owner-id`). THE ROOT CAUSE OF THE 2026-09-04 INCIDENT:
+  # neither cluster set it, so both inherited the chart default `default`, and ExternalDNS treats a
+  # record whose TXT registry entry carries ITS OWN owner id as its own to reconcile — and with
+  # `policy: sync`, to DELETE when no local resource claims it. Two clusters sharing an owner id on
+  # one zone therefore garbage-collect each other. Each environment MUST have a distinct value.
+  #
+  # DEV MUST RENDER EXACTLY `default`. Dev's LIVE public records are already registered in Cloudflare
+  # under owner `default`; changing dev's value would make dev's own external-dns stop recognising
+  # them, and it would re-create duplicates alongside the orphaned originals. Dev's overlay pins it.
+  #
+  # THE BASE DEFAULT IS A NON-COLLIDING SENTINEL, deliberately NOT `default`. A new environment that
+  # forgets this knob must not silently inherit dev's identity — that is precisely the incident. With
+  # this sentinel such a cluster owns nothing, so it can DELETE nothing; it would publish under a
+  # visibly wrong owner id (grep the TXT records) instead of destroying a live environment's DNS.
+  EXTERNALDNS_TXT_OWNER_ID: "unset-see-cluster-vars"
+
   ACME_CLUSTER_ISSUER: "letsencrypt-staging" # dev: staging | prod: letsencrypt-prod (ADR-0006)
   GATEWAY_VIP: "192.168.1.53"                # in-cluster split-horizon target (R-DNS-01/03): the L2-announced
                                             # ingress VIP of the `shared` Gateway. MUST equal the FIRST address of