docs: Unraid as a Herdr machine, and why it needed a --via wrapper

herdr-core machine add kept failing with "core did not start within 5 s".
Cause: hc puts its core socket under XDG_RUNTIME_DIR, which on Unraid is
/run/user/0 — created and destroyed per login session by elogind. The socket
vanished with the session that started the core, so later clients found an
empty directory. The core itself kept running on an unlinked socket, and
core.log stayed empty because the failure is client side.

Fixed entirely on the Mac with a --via transport that pins
XDG_RUNTIME_DIR=/var/run/herdr-rt. Nothing on the server was changed. The
transport uses key auth on 192.168.10.20:422, not Tailscale SSH, since --via
needs something that asks nothing and the default tailnet ACL demands
periodic browser re-auth.

Also records the reboot caveat (/ is RAM, so the remote hc binary is lost)
with a go-script fix written up but not applied, and corrects the
192.168.31.0/24 note: that shim was created for a Pi-hole that AdGuard
replaced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/CHANGELOG.md

@@ -13,6 +13,12 @@
 - **[SERVICE]** Exit node advertised from Unraid as a fallback to the hAP, and approved in the Tailscale admin console the same day. The tailnet now has two exit nodes: `mikrotik-tailscale` (100.74.219.35, primary) and `xtrm-unraid` (100.100.208.69, fallback). Neither has been exercised in anger from the Mac — without `--exit-node-allow-lan-access` an exit node cuts the client's own LAN access.
 - Detail, including the root-only `AllowTcpForwarding` behaviour in Unraid's stock sshd config, in `docs/09-TAILSCALE-VPN.md`.
 
+### Tooling: Unraid registered as a Herdr (hc) machine
+- **[SERVICE]** Unraid added to Herdr as the machine `unraid` (`hc` 0.1.17 on the remote). `herdr-core machine add` had been failing with `core did not start within 5 s`: `hc` puts its core socket under `XDG_RUNTIME_DIR`, which on Unraid is `/run/user/0` — a directory elogind creates and destroys per login session, so the socket disappeared the moment the session that started the core ended. The core kept running with an unlinked socket, and `core.log` stayed empty because the failure is client side.
+- **[SERVICE]** Fixed from the Mac only, via `~/.local/bin/herdr-via-unraid`, a `--via` transport that exports `XDG_RUNTIME_DIR=/var/run/herdr-rt` on every connection. Nothing on the server was changed. Uses key auth on 192.168.10.20:422 rather than Tailscale SSH, because `--via` needs a transport that asks nothing and the default tailnet ACL `check` would demand periodic browser re-auth.
+- **[SERVICE]** Reboot caveat: `/` is rootfs (RAM) and `/run` is tmpfs on Unraid, so `/root/.local/bin/herdr-core` is lost at every boot and needs `herdr-core machine update` to restore. A `/boot/config/go` entry to restore it automatically is written up but **not applied**. Detail in `docs/14-HERDR-REMOTE-MACHINES.md`.
+- Corrected the 192.168.31.0/24 note: the shim was created in the go script for a Pi-hole at 192.168.31.4, which AdGuard replaced. No such container exists and the address is silent.
+
 ---
 ## 2026-10-16
 - Home Assistant: `corridor-bt-proxy` (Olimex ESP32-POE-ISO) added to the ESPHome integration and running as an active Bluetooth proxy. Temporarily on hAP ax3 ether3, VLAN 10 (192.168.10.100); it moves to CAP XL ether2 (VLAN 30) when mounted.