Auto-deploy skillset on container start for portable skill resolution

Add entrypoint logic to detect and run the skillset deploy script on
container start. Detection order: SKILLSET_CONTAINER_PATH env var,
then ~/skillset dedicated mount, then /workspace/skillset fallback.

The deploy script (from the skillset repo) creates relative symlinks
that resolve inside the container regardless of the host path layout.

Also adds SKILLSET_PATH volume mount option to docker-compose files
and documents SKILLSET_CONTAINER_PATH in .env.example for hosts where
the skillset lives in a workspace subdirectory.

docker-compose.shared.yml

@@ -45,6 +45,12 @@ services:
       # SSH keys — user-specific if available, else shared
       - ${SSH_KEY_PATH:-~/.ssh}:/home/developer/.ssh:ro
 
+      # Optional: mount skillset repo for automatic skill/instruction deployment.
+      # The entrypoint runs deploy-skills.sh --bootstrap on start, creating
+      # relative symlinks that resolve inside the container regardless of
+      # 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