fix(vpn-ingress): keep the server key in source through SOPS, not as an out-of-band Secret

Operator correction: secrets are kept in source through sops. #55 shipped the
WireGuard server private key as a Secret to `kubectl create` by hand. Two real
costs, not just a convention break:

  * R-SEC-01 / spec docs/07 -- secrets live in source, SOPS-encrypted, and
    reach the cluster only through Flux.
  * It broke DR for the one credential that cannot be re-derived. A rebuild
    restores everything in git; it does not restore a hand-made Secret, and
    losing this key means re-enrolling EVERY device, because each client
    profile pins its public half. #55 papered over that with "keep a copy in
    your password manager".

(Note secret-provenance did NOT forbid the original shape -- it scopes to
app-namespace=true and vpn-ingress is deliberately a platform namespace. The
rule broken was the written requirement, not an enforced control.)

An agent still must not mint a long-lived private key (CLAUDE.md §6), so the
file is committed ALREADY ENCRYPTED with `privatekey: REPLACE-ME`. That makes
the wiring complete and green from the first commit -- resource line plus both
overlay decryption patches -- leaving the operator exactly one act: replace the
value. A commented-out resource to uncomment later is the "in git, no generator
entry, reviewed, merged, still broken" shape this repo keeps hitting.

The init guard is now a validation rather than an emptiness check: the value is
piped through `wg pubkey`, which accepts only a real curve25519 private key, and
the pod names which case it hit -- placeholder, empty, mangled, or (the silent
one) still `ENC[` ciphertext because an overlay is missing its decryption patch,
which otherwise applies cleanly with every Flux condition green.

Second, independent finding that also fixes #55's step 2: `wg` is on the HOST
only and `sops` is in the CONTAINER only, so `sops -d | wg pubkey` cannot run in
one shell -- the command #55 documented could not have worked. Every step is now
split by where it runs, and the written key is verified by comparing sha256
digests rather than printing anything. `qrencode` is on neither, so the QR step
says so and gives the install command.

Touches clusters/** and adds a **/*.sops.yaml, so approval-policy will block
pending the operator's /approve-override. Deliberately not posted by me.

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

clusters/dev/kustomization.yaml

@@ -122,6 +122,24 @@ patches:
           provider: sops
           secretRef:
             name: sops-age
+  # vpn-ingress (the WireGuard remote-access VPN, clusters/base/vpn-ingress.yaml) carries
+  # platform/vpn-ingress/wg-server-key.sops.yaml — the WireGuard SERVER private key, the one
+  # secret in that component. Without this stanza kustomize hands Flux the raw ciphertext and the
+  # Secret applies full of ENC[...]: the APPLY SUCCEEDS and every Flux condition stays green, and
+  # the only symptom is a VPN that no client can handshake with. The DaemonSet's init container
+  # validates the value with `wg pubkey` specifically so that state is a named CrashLoop instead.
+  # The `sops-age` Secret is out-of-band (name only).
+  - patch: |-
+      apiVersion: kustomize.toolkit.fluxcd.io/v1
+      kind: Kustomization
+      metadata:
+        name: vpn-ingress
+        namespace: flux-system
+      spec:
+        decryption:
+          provider: sops
+          secretRef:
+            name: sops-age
   # pm-agent (the Program Manager agent service, clusters/base/pm-agent.yaml) carries
   # pm-agent-credentials.sops.yaml — the Gitea write token for the dedicated non-admin bot
   # account and the Anthropic subscription token from `claude setup-token`. Without this