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]>

platform/externaldns/helmrelease.yaml

@@ -29,15 +29,37 @@ spec:
     sources:
       - gateway-httproute
       - service
-    # domain-filter MUST be the actual Cloudflare ZONE (keithjackson.dev), not the platform subdomain
-    # (home.keithjackson.dev) — ExternalDNS filters the account's zones by this value, and a subdomain
-    # filter drops the parent zone → "no hosted zone matching record DNS Name" and nothing is created.
-    # Scope stays effectively home.keithjackson.dev because that's the only suffix services request;
-    # policy=sync + registry=txt only ever touch ExternalDNS-OWNED records, never the pre-existing
-    # wildcard or other keithjackson.dev records. (docs/09; resolves the WS-NET TODO below.)
+    # ---------------------------------------------------------------------------------------------
+    # CROSS-CLUSTER DNS OWNERSHIP. Read before changing either knob below.
+    #
+    # On 2026-09-04 prod's external-dns DELETED EVERY ONE of dev's public records. Both clusters ran
+    # `policy: sync` against the SAME Cloudflare zone, served the SAME hostnames, and — because
+    # NEITHER set txtOwnerId — both inherited the chart default owner id `default`. Prod therefore
+    # read dev's TXT registry entries as its own, found no local resource claiming those names, and
+    # garbage-collected them. `policy: sync` is correct and stays; what was missing was identity.
+    # Full write-up: infra/PROD-PROVISIONING.md §10.
+    #
+    # TWO INDEPENDENT GUARDS NOW, either of which alone prevents a recurrence:
+    #   1. txtOwnerId       — per-environment identity; a cluster only ever touches records it owns.
+    #   2. domainFilters    — per-environment name scope; prod cannot even see names outside its
+    #                         own subtree. Defence in depth, added after the incident.
+    # ---------------------------------------------------------------------------------------------
+    # Which names this instance may manage at all. Cloudflare-provider caveat, empirically hit
+    # 2026-08: this value is used for ZONE selection as well as record matching, and a filter that is
+    # a strict subdomain of the zone dropped the parent zone entirely ("no hosted zone matching record
+    # DNS Name") — so DEV keeps the zone apex `keithjackson.dev`, which is also what dev serves the
+    # apex of. PROD narrows to `prod.home.keithjackson.dev`. If prod's first reconcile logs no
+    # matching zone, that caveat still applies: prod creates NOTHING (fail-closed, never destructive)
+    # and the fallback is to widen prod's EXTERNALDNS_DOMAIN_FILTER back to the zone apex, leaving
+    # txtOwnerId as the sole guard. See clusters/base/cluster-vars.yaml.
     domainFilters:
-      - "${DNS_ZONE}"
-    policy: sync            # create + update + delete records as services come and go
+      - "${EXTERNALDNS_DOMAIN_FILTER}"
+    # The TXT-registry owner id. MUST be distinct per environment — this is the knob whose absence
+    # caused the incident. Dev renders `default` (its live records are already registered under that
+    # owner; changing it would orphan them and duplicate every record). Prod renders `prod`.
+    txtOwnerId: "${EXTERNALDNS_TXT_OWNER_ID}"
+    policy: sync            # create + update + delete records as services come and go — but ONLY
+                            # records this owner id owns AND this domainFilter admits (see above).
     # Cloudflare API token: SOPS-encrypted secret, referenced BY NAME only (OQ-4, ADR-0007).
     env:
       - name: CF_API_TOKEN