Document the upgrade-ritual for reconciling VM compose files

New releases may add named volumes or bind-mount lines to
docker-compose.yml. The image can't update compose files on the VM —
they're user-owned — so a plain 'docker compose pull && up -d' picks
up the new image but silently misses new mount points.

Example from v1.14.19c → v1.14.20: bash history persistence needs
the devbox-shell-history named volume at /home/developer/.cache/bash.
The v1.14.20 image is configured to write history there either way,
but without the volume mount on the VM, writes land in the container's
writable layer and vanish on every --force-recreate.

Add a 'Upgrading an existing VM to a new release' subsection to
deploy/README.md describing the backup → diff → merge → recreate
ritual, so future upgrades don't quietly drop features the same way.

deploy/README.md

@@ -198,6 +198,34 @@ After editing `docker-compose.yml` on the VM to uncomment the bind mounts you ne
 
 The script reads `docker-compose.yml` on the remote VM, detects which bind mounts are active, and syncs only those directories from your local machine. It also creates the remote directories if they don't exist.
 
+### Upgrading an existing VM to a new release
+
+Each tagged release may add new named volumes or bind-mount lines to `docker-compose.yml`. Pulling a new image via `docker compose pull` grabs the new container behaviour, but compose files on the VM are user-owned and never touched by the image — you have to reconcile them yourself when upgrading across versions.
+
+**Symptom of a missed reconcile:** a new feature quietly doesn't work even though the image is correct. Example from v1.14.19c → v1.14.20: bash history persistence requires the `devbox-shell-history` named volume mounted at `/home/developer/.cache/bash`. The v1.14.20 image writes history to that path either way, but without the volume mount on the VM, writes land in the container's writable layer and vanish on every `--force-recreate`.
+
+**Upgrade ritual:**
+
+```bash
+# On the VM, before recreating the container:
+cd ~/opencode-devbox
+cp docker-compose.yml docker-compose.yml.bak-$(date +%Y%m%d-%H%M%S)
+
+# Compare against the repo version to see what's new:
+#   (from your local checkout)
+scp devbox-affection:~/opencode-devbox/docker-compose.yml /tmp/vm-compose.yml
+diff -u /tmp/vm-compose.yml ~/src/src_local/opencode-devbox/docker-compose.yml
+```
+
+For each new `volumes:` entry or mount line in the repo version that isn't in your VM's file, add it manually — preserving any local customizations you've made (image variant, read/write flags on bind mounts, etc.). Then:
+
+```bash
+docker compose config >/dev/null   # verify YAML still parses
+docker compose up -d --force-recreate
+```
+
+If you maintain the VM's compose file with no local changes, `scp` the repo version over wholesale. If you have customizations (the common case), do the diff-and-merge by hand.
+
 ### Shell defaults inside the container
 
 The image ships baked `.bash_aliases` and `.inputrc` in `/etc/skel-devbox/` — quality-of-life defaults (prefix history search on Up/Down arrows, persistent history across container recreates via the `devbox-shell-history` named volume, `[devbox]` prompt marker, sensible aliases). On first container start the entrypoint copies them to `/home/developer/` **only if the target file does not already exist**.