Initialize duroxide-pg-opt during Codespaces setup

[pinodeca]

Summary

• initialize the duroxide-pg-opt submodule during devcontainer onCreate setup
• document that Codespaces can authenticate to the private submodule via the configured repository permission
• degrade gracefully outside Codespaces when the private submodule is not accessible

Validation

• bash -n .devcontainer/onCreateCommand.sh
• bash -n .devcontainer/postCreateCommand.sh

[pinodeca]

Abandoning this PR. The Codespace repo permissions don't work during Prebuild/onCreateCommand. This PR will be superseded by #92 which will use a PAT instead so that duroxide-pg-opt submodule can be initialized and the pg_durable can be built - during Prebuild.