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).

CHANGELOG.md

@@ -8,7 +8,17 @@ Tags follow the pi npm version: `v{pi_version}[letter]` — bare tag for the fir
 
 ## Unreleased
 
-_(no changes since v0.78.0c)_
+### Added: persist the LAN-jump key + one-line authorize hint
+
+- **compose:** persist `~/.ssh-local` via a new `devbox-ssh-local` named volume
+  so the generated LAN-jump key survives `docker compose up --force-recreate`.
+  You authorize the key on the host **once per machine** instead of after every
+  container update.
+- **Inherited from base:** `setup-lan-access.sh` now prints a copy-paste
+  `echo '…' >> ~/.ssh/authorized_keys` line when it generates a new key
+  (published via opencode-devbox's `base-pi-only`). No helper file to locate.
+
+_(no other changes since v0.78.0c)_
 
 ## v0.78.0c — 2026-06-04