fix(opencode): bind workspace volumes to ZFS on CT 116

Keep repos, builds, and worktrees on apps/submatter/* (~890G) instead
of the thin-provisioned 48G root disk.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker/docker-compose.prod.yml

@@ -305,8 +305,20 @@ services:
       - echodo
 
 volumes:
+  # Backed by ZFS bind mounts on CT 116 (apps/submatter/* → /mnt/*).
+  # Repos, worktrees, and builds grow here — not on the 32G local-lvm root.
   echodo-workspace:
+    driver: local
+    driver_opts:
+      type: none
+      o: bind
+      device: /mnt/workspace
   opencode-data:
+    driver: local
+    driver_opts:
+      type: none
+      o: bind
+      device: /mnt/opencode-data
 
 networks:
   echodo: