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/prod/kustomization.yaml

@@ -201,6 +201,63 @@ patches:
       - op: replace
         path: /data/ACME_CLUSTER_ISSUER
         value: "letsencrypt-prod"
+      # ==========================================================================================
+      # PROD DNS IDENTITY (R-DNS-01/02/03) — the fix for the 2026-09-04 incident.
+      # ==========================================================================================
+      # PROD SERVES ITS OWN SUBDOMAIN. Until 2026-09-04 every hostname was HARDCODED to
+      # home.keithjackson.dev in the base manifests, so prod was authored to serve the EXACT SAME
+      # public names as the live dev cluster out of the SAME Cloudflare zone. Two consequences, one
+      # of which already happened:
+      #   1. HAPPENED: prod's external-dns (policy: sync, owner id `default` — the same implicit
+      #      default dev had) concluded it owned dev's records and DELETED EVERY ONE of them.
+      #   2. QUEUED BEHIND IT: platform-config would have requested a `*.home.keithjackson.dev`
+      #      wildcard from letsencrypt-prod for a cert dev already holds, and both clusters would
+      #      have raced each other's `_acme-challenge` TXT records on that one zone.
+      # Moving prod to its own suffix dissolves both: the two clusters can no longer name the same
+      # record. This is what makes ACME_CLUSTER_ISSUER: letsencrypt-prod above safe here.
+      #
+      #   dev   -> home.keithjackson.dev         (UNCHANGED — dev is live; phase 2 moves it)
+      #   prod  -> prod.home.keithjackson.dev    (this knob)
+      #
+      # CONSEQUENCE — prod's hostnames are NOT the apex names. Prod's Gitea is
+      # gitea.prod.home.keithjackson.dev, its Keycloak keycloak.prod.home.keithjackson.dev, and so
+      # on; the wildcard cert becomes `*.prod.home.keithjackson.dev`. The apex CNAMEs that make
+      # `<svc>.home.keithjackson.dev` point at prod are PHASE 2, deliberately not done here — they
+      # would take traffic off the live dev cluster. infra/PROD-PROVISIONING.md §10.
+      #
+      # [parity] MUST equal k3s_base_domain in infra/ansible/inventory/group_vars/k3s_prod/vars.yml.
+      # The kube-apiserver's oidc-issuer-url is an INSTALL-TIME k3s flag, not a manifest, so it is
+      # set Ansible-side from that variable and cannot read this one. If they disagree, kubectl SSO
+      # trusts a different Keycloak than the one prod actually runs.
+      - op: replace
+        path: /data/BASE_DOMAIN
+        value: "prod.home.keithjackson.dev"
+      # SAFETY NET #2 (independent of the owner id below): narrow ExternalDNS's name scope to prod's
+      # OWN subtree, so prod cannot create, update or delete any record outside it no matter what the
+      # TXT registry says. VERIFY ON FIRST RECONCILE — the Cloudflare provider uses this value for
+      # ZONE selection too, and on the version in use in 2026-08 a strict-subdomain filter dropped
+      # the parent zone ("no hosted zone matching record DNS Name"). If prod's external-dns logs no
+      # matching zone, it will create NOTHING — fail-closed, and it can still never delete dev's
+      # records. Fallback in that case: set this to "keithjackson.dev" (dev's value) and rely on
+      # EXTERNALDNS_TXT_OWNER_ID below, which is sufficient on its own.
+      - op: replace
+        path: /data/EXTERNALDNS_DOMAIN_FILTER
+        # MUST be the Cloudflare ZONE, not prod's subdomain. FINDING F-DNS1 (DECISIONS-LOG,
+        # 2026-08-17) proved a subdomain filter DROPS THE PARENT ZONE: ExternalDNS generated the
+        # record then skipped it with "no hosted zone matching record DNS Name", so prod would
+        # publish NOTHING. Isolation from dev does NOT come from this filter — it comes from
+        # (1) prod only requesting *.prod.home.keithjackson.dev hostnames, and
+        # (2) txtOwnerId: prod, so prod's registry never claims dev's `default`-owned records.
+        # That pairing is what the 2026-09-04 incident needed and this filter cannot provide.
+        value: "keithjackson.dev"
+      # SAFETY NET #1 — THE ACTUAL ROOT CAUSE OF THE INCIDENT. Both clusters left this unset and so
+      # both inherited the chart default `default`; prod then treated dev's TXT-registry entries as
+      # its own and, under policy: sync, deleted the records behind them. This value MUST differ from
+      # dev's (`default`, pinned in clusters/dev — dev's live records are registered under it and
+      # cannot be renamed without orphaning them). `prod` is distinct, which is the entire fix.
+      - op: replace
+        path: /data/EXTERNALDNS_TXT_OWNER_ID
+        value: "prod"
       - op: replace
         path: /data/LONGHORN_DEFAULT_REPLICA_COUNT
         value: "3"