Pin project name in default docker-compose.yml

Without an explicit name, Docker Compose derives the project name
from the directory basename. Renaming the directory silently orphans
all named volumes (devbox-data, devbox-state, devbox-shell-history,
etc.) because the new project name no longer matches the old volume
prefixes. Pin to 'opencode-devbox' so volumes survive directory
moves and renames.

docker-compose.yml

@@ -8,6 +8,11 @@
 # Or for interactive one-shot:
 #   docker compose run --rm devbox
 
+# Pin the project name so named volumes survive directory renames.
+# Without this, Docker Compose derives the project name from the
+# directory basename — renaming the dir orphans all existing volumes.
+name: opencode-devbox
+
 services:
   devbox:
     image: joakimp/opencode-devbox:latest