Use named volume for opencode config instead of host bind mount

Switching from a host bind mount (~/.config/opencode) to a named volume
(devbox-opencode-config) eliminates the symlink conflict between host
and container environments. Each manages its own skill/instruction
symlinks independently, allowing native opencode and containerized
opencode to coexist on the same machine.

Also removes the ~/.agents/skills bind mount recommendation — the
container manages its own skills directory via the entrypoint deploy,
and sharing it with the host causes relative-path conflicts.

docker-compose.shared.yml

@@ -51,8 +51,12 @@ services:
       # where the repo lives on the host. Set SKILLSET_PATH in .env.
       # - ${SKILLSET_PATH}:/home/developer/skillset
 
-      # Opencode config — per-user (persists settings across restarts)
-      - ${HOME}/${SIGNUM}/.config/opencode:/home/developer/.config/opencode
+      # Persist opencode config (opencode.jsonc, oh-my-opencode-slim.json,
+      # instructions, etc.) across container recreations. Auto-generated on
+      # first start from env vars by generate-config.py and the skillset
+      # deploy script. Using a named volume keeps the container's symlinks
+      # independent from the host.
+      - devbox-opencode-config:/home/developer/.config/opencode
 
       # Persist opencode data (auth, memory, session history)
       - devbox-data:/home/developer/.local/share/opencode
@@ -79,6 +83,7 @@ services:
       # - ${HOME}/${SIGNUM}/.aws:/home/developer/.aws
 
 volumes:
+  devbox-opencode-config:
   devbox-data:
   devbox-shell-history:
   devbox-zoxide: