V3 Yarn toolchain capability pack. By-value connector composition (service_kind=cli, no separate ingredient): one local yarn CLI ingredient exposes bounded project operations for readiness and package test execution. Project operations run from an explicit project directory cwd supplied at execution time, avoid caller-supplied Yarn flags, avoid shell wrappers, and keep diagnostics visible as text output. Test execution is approval-gated because package scripts execute project-defined code and Yarn lifecycle behavior. The pack exposes no caller-supplied script name, package-manager flags, workspace selectors, install/update/audit command, registry, env override, stdin, or arbitrary command execution. Requires yarn on PATH.