Fix volume collision in shared-machine compose: scope project name by SIGNUM

The Docker daemon is system-wide — named volumes are prefixed by the
compose project name, which defaults to the basename of the directory
holding docker-compose.yml. Two users whose compose file lives under
a directory with the same name (e.g. ~/alice/opencode-devbox and
~/bob/opencode-devbox) would silently share volumes, corrupting each
other's opencode data, bash history, and TUI settings.

Add an explicit top-level 'name: devbox-${SIGNUM}' so the project
name (and therefore all volume prefixes) is unique per user. The old
comment claiming directory-name prefixing was sufficient was wrong —
it only works if directory basenames differ, which isn't guaranteed
on multi-user hosts or when users follow the same setup instructions.

docker-compose.shared.yml

@@ -12,9 +12,13 @@
 #   5. mkdir -p ~/<signum>/.config/opencode
 #   6. docker compose up -d
 #
-# Named volumes are automatically isolated per user because Docker Compose
-# prefixes them with the project directory name (e.g. opencode-devbox_devbox-data).
-# Since each user runs from ~/<signum>/opencode-devbox/, volumes don't collide.
+# Volume isolation: the top-level 'name:' field uses SIGNUM to derive a
+# unique project name (devbox-<signum>), which Docker Compose uses as the
+# prefix for all named volumes. Without this, two users whose compose file
+# lives in a directory with the same basename would share volumes — the
+# Docker daemon is system-wide and doesn't scope by OS user.
+
+name: devbox-${SIGNUM:?Set SIGNUM in .env}
 
 services:
   devbox: