feat(studio): unify dev sandbox and add system info

[evanlowe]

Summary

• add a System Information page for the Studio version, configured sandbox tools, and the current Identity user pool, including local and empty states
• consolidate Skill generation and refinement on the shared Dev Sandbox, and remove the legacy Skill Creator entry, implementation, and CLI option
• align Studio deploy/update sandbox configuration, documentation, tests, and bundled frontend assets

Testing

• .venv/bin/pre-commit run
• cd frontend && npm test (433 passed)
• cd frontend && npm run build
• .venv/bin/pytest -q tests/cli/test_frontend_sandbox_options.py tests/cli/test_studio_deploy_target.py tests/cli/test_studio_rbac.py tests/cli/test_studio_update.py (106 passed)