ADR-0015: VPN egress steering — node-level tunnel + Cilium Egress Gateway

The in-cluster Gluetun-pod gateway (ADR-0013) cannot be steered: a pod's tun0 is
invisible to the node datapath, and hostNetwork Gluetun hijacks the node's own
default route (breaks k3s/etcd). The tunnel itself works (PIA egress validated),
but not the steering.

New decision (ADR-0015): move the tunnel to a HOST OpenVPN systemd service on
egress-gateway node(s) (Ansible-managed, split-tunnel so it doesn't touch the node
default route), steer egress-class=vpn pods with a CiliumEgressGatewayPolicy (native
eBPF, no client-pod changes), and enforce the kill-switch with a host nftables rule
(fail-closed independent of Cilium). PIA/OpenVPN-UDP provider decision (ADR-0013)
stands; the .ovpn transfers over; PF re-solved host-side (Phase 6).

Amended for consistency: ADR-0013 (client+steering superseded), decisions/README,
docs/00, docs/01, docs/03, docs/07, agents/task-board.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

docs/01-architecture-overview.md

@@ -15,7 +15,7 @@ choice here without a superseding ADR.**
 | VM workloads (IaaS) | **KubeVirt** (+ CDI) | R-PLAT-03 | [ADR-0005](../decisions/adr-0005-vm-workloads-kubevirt.md) | [06](06-vm-workloads-kubevirt.md) |
 | TLS | **cert-manager** + Let's Encrypt (DNS-01) | R-TLS-01..03 | [ADR-0006](../decisions/adr-0006-tls-cert-manager-letsencrypt.md) | [08](08-tls-and-certificates.md) |
 | Secrets | **SOPS + age** (Flux-native) | R-SEC-01 | [ADR-0007](../decisions/adr-0007-secrets-sops-age.md) | [07](07-security-and-secrets.md) |
-| VPN egress | **Single shared in-cluster PIA/Gluetun gateway** (OpenVPN-UDP + native PF) + Cilium Egress Gateway | R-NET-05/06/07 | [ADR-0013](../decisions/adr-0013-vpn-provider-pia-gluetun-wireguard.md) (supersedes 0012/0008) | [03](03-networking-isolation-and-egress.md) |
+| VPN egress | **Node-level PIA/OpenVPN tunnel** steered by **Cilium Egress Gateway** (host nftables kill-switch) | R-NET-05/06/07 | [ADR-0015](../decisions/adr-0015-vpn-egress-steering-node-tunnel-cilium.md) (provider [0013](../decisions/adr-0013-vpn-provider-pia-gluetun-wireguard.md)) | [03](03-networking-isolation-and-egress.md) |
 | Automated updates | **Flux image-update-automation** (images) + **Renovate** (charts) | R-UPD-01..03 | [ADR-0009](../decisions/adr-0009-automated-image-updates.md) | [11](11-automated-updates.md) |
 | DNS records | **ExternalDNS** (public via Cloudflare) + **PiHole** (internal) | R-DNS-01..03 | — | [09](09-dns-and-ingress.md) |
 | Ingress (north-south) | **Gateway API** implemented by Cilium | R-DNS-01, R-TLS-01 | [ADR-0002](../decisions/adr-0002-cni-cilium.md) | [09](09-dns-and-ingress.md) |
@@ -46,7 +46,7 @@ choice here without a superseding ADR.**
 │  Storage: Longhorn (replicated block)  +  bulk media store (see ADR-0003)  │
 ├──────────────────────────────────────────────────────────────────────────┤
 │  Network: Cilium  (default-deny NetworkPolicy · Egress Gateway · Hubble)   │
-│           VPN egress (Gluetun sidecars / VPN gateway) · Multi-WAN routing  │
+│           VPN egress (node tunnel + Cilium egress GW) · Multi-WAN routing  │
 ├──────────────────────────────────────────────────────────────────────────┤
 │  Kubernetes: k3s (3 nodes, HA control plane, embedded etcd)                │
 ├──────────────────────────────────────────────────────────────────────────┤