Create an Instance AI workflow eval

Each eval is one JSON case — authored locally as a file in packages/@n8n/instance-ai/evaluations/data/workflows/ (the disk loader auto-discovers *.json, no registration step), with a LangTracer suite as its durable home. Cases validate against harness/schema.ts (.strict() — unknown keys fail at load). The eval README is the exhaustive field reference; this skill is the opinionated how.