compose: persist LAN jump key (devbox-ssh-local volume) + docs

Persist ~/.ssh-local so the generated LAN-jump key survives container
recreation; authorize it on the host once per machine. Adds the volume
to the compose template and documents it in the README volumes table.
LAN-access mechanism/script changes are inherited from base-pi-only
(opencode-devbox).

docker-compose.yml

@@ -49,6 +49,12 @@ services:
       # Persist pi config (settings.json, extensions, sessions, auth)
       - devbox-pi-config:/home/developer/.pi
 
+      # Persist the generated LAN-jump keypair (~/.ssh-local) across recreates.
+      # setup-lan-access.sh generates this key once and reuses it; persisting
+      # it means you authorize it on the host ONCE rather than re-authorizing
+      # after every `docker compose up --force-recreate`.
+      - devbox-ssh-local:/home/developer/.ssh-local
+
       # Persist bash history across container recreations
       - devbox-shell-history:/home/developer/.cache/bash
 
@@ -72,6 +78,7 @@ services:
 
 volumes:
   devbox-pi-config:
+  devbox-ssh-local:
   devbox-shell-history:
   devbox-zoxide:
   devbox-nvim-data: