fix(vpn-ingress): stop trying to set net.ipv4.ip_forward — a hostNetwork pod cannot, and wg-quick dies on a failed PostUp

The tunnel never came up on its first reconcile with a real key. One line of my
own manifest was the whole cause. Measured on prod 2026-10-06:

    [#] ip link add dev wg0 type wireguard        <- fine
    [#] wg addconf wg0 /dev/fd/63                 <- fine
    [#] ip -4 address add 10.90.0.1/24 dev wg0    <- fine
    [#] ip link set mtu 1420 up dev wg0           <- fine
    [#] sysctl -q -w net.ipv4.ip_forward=1
    sysctl: permission denied on key "net.ipv4.ip_forward"
    [#] ip link delete dev wg0
    **** Tunnel failed, will stop all others! ****

Not a missing capability: with hostNetwork the runtime mounts /proc/sys
READ-ONLY (confirmed in the pod's /proc/mounts), because those sysctls are the
HOST's globals. CAP_NET_ADMIN cannot lift a read-only mount, so the write can
never succeed here -- and wg-quick treats a failed PostUp as fatal, so it
deleted the interface it had just built.

The line was never needed. node_baseline already sets ip_forward=1, and reading
it from the pod returns 1. My header comment said exactly that ("so no pod
sysctl is needed") and then emitted the sysctl anyway, justified as "re-asserted
so this component does not silently depend on another role's sysctl file". That
defensive re-assertion is the entire outage. In a hostNetwork pod: VERIFY host
state, never write it.

Replaced with a read in the init container -- `grep -qx 1
/proc/sys/net/ipv4/ip_forward` -- failing closed with a message naming the
node_baseline role. Still fail-closed on purpose: a tunnel that comes up unable
to forward satisfies every probe here and silently drops all client traffic.

Also documents, in the manifest and the runbook, that the image's `chown:
Operation not permitted` run and its "we will not provide support for it"
warning are expected and harmless -- capabilities are dropped to NET_ADMIN so
its s6 init has no CAP_CHOWN, /config is a writable tmpfs, everything runs as
root, and wg-quick proceeds (it did; the failure was 30 lines later).

Three other unknowns closed by the same reconcile, now recorded in the README:
the kernel module reaches the pod (ip link add type wireguard works, no
CAP_SYS_MODULE needed); allowPrivilegeEscalation:false does NOT break the s6
init; and the image's iptables is nf_tables (xtables-nft-multi) matching Debian
13, which closes the worst documented fear -- rules landing in a table the
kernel never consults.

No sensitive paths; merges on a green gate with no override.

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

runbooks/vpn-remote-access.md

@@ -134,6 +134,36 @@ specific failure in its log — so you never get a server that silently cannot c
 | `It is EMPTY` | the value was blanked |
 | `still SOPS CIPHERTEXT (starts with ENC[)` | the Secret reached the cluster undecrypted — this environment's overlay is missing its `vpn-ingress` decryption patch. **This is the dangerous one:** the Secret applies cleanly and every Flux condition stays green |
 | `wg pubkey rejected it` with none of the above | a mangled value, or a *public* key pasted by mistake |
+| `net.ipv4.ip_forward is 0 on this node` | the node_baseline role has not run here (step 1). The pod **cannot** set this itself — under `hostNetwork` the runtime mounts `/proc/sys` read-only, because it is the host's global setting |
+
+### What a healthy start looks like
+
+Measured on prod 2026-10-06. The init container logs one line, naming the shape and never the key:
+
+```
+wg0.conf assembled: 0 peer(s), server 10.90.0.1/24, udp/51820.
+```
+
+then the `wireguard` container brings the tunnel up:
+
+```
+**** Found WG conf /config/wg_confs/wg0.conf, adding to list ****
+**** Activating tunnel /config/wg_confs/wg0.conf ****
+[#] ip link add dev wg0 type wireguard
+[#] wg addconf wg0 /dev/fd/63
+[#] ip -4 address add 10.90.0.1/24 dev wg0
+[#] ip link set mtu 1420 up dev wg0
+```
+
+**Ignore the `chown: ... Operation not permitted` run before that**, and the image's own
+"Permissions could not be set ... we will not provide support for it". Capabilities are dropped to
+`NET_ADMIN` only, so its s6 init has no `CAP_CHOWN`; `/config` is a writable tmpfs, everything runs
+as root, and `wg-quick` proceeds. It is wrong about the cause and it does not matter.
+
+**A pod stuck `0/1 Running` with `Startup probe failed: Unable to access interface: No such device`
+means `wg-quick` tore the tunnel down** — read the `wireguard` container log for the `[#]` line it
+died on. That is the shape of a real failure; the probe is doing its job of refusing to call a
+server without a `wg0` "ready".
 
 ## Step 3 — forward udp/51820 on the router (once)