Add help-osk-docs stack: Zensical instructions site for 05-osk

Static instructions site for client 05 served at help.osk.team. Markdown +
images built by Zensical (zensical build) into public/, served by nginx behind
Traefik. Content is bind-mounted from /mnt/containers/help-osk-docs (absolute
host path — Portainer GitOps can't resolve relative mounts). Adds the
help/help-osk router+service to traefik/dynamic/web.yml (TLS via Let's Encrypt,
public/open).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

traefik/dynamic/web.yml

@@ -15,6 +15,13 @@ http:
       tls:
         certResolver: le
 
+    help:                       # client 05 instructions site (public, open)
+      entryPoints: ["websecure"]
+      rule: "Host(`help.osk.team`)"
+      service: help-osk
+      tls:
+        certResolver: le
+
     portainer:
       entryPoints: ["websecure"]
       rule: "Host(`portainer.osk.team`)"
@@ -35,6 +42,10 @@ http:
       loadBalancer:
         servers:
           - url: "http://flame:5005"
+    help-osk:
+      loadBalancer:
+        servers:
+          - url: "http://help-osk:80"   # nginx container on reverseproxy-nw
     portainer:
       # Portainer must share a network with Traefik. Either add the portainer
       # container to reverseproxy-nw, or point this at the host IP instead: