Add explicit dev asset root mode

[jsugg]

Summary

• default runtime asset resolution to packaged assets unless an explicit asset override or STRONGCLAW_ASSET_ROOT is provided
• add repo-local developer entrypoints with scripts/dev-env.sh, make dev-shell, and bin/clawops-dev for repo-backed asset workflows
• cover the new asset-root behavior and developer entrypoints with focused unit and contract tests while updating contributor docs