metric-groups create over-requires projects/description (spec/back-end)

**Behavior:** `postMetricGroup` specs `projects` and `description` as required (no sibling create does), so `metric-groups create` demands both flags; an org-wide group needed `--body '{"projects":[],...}'` because a stringArray flag couldn't express `[]`.

**Mitigation in place:** the stringArray JSON-expansion patch (PR #48, tracked in #51) makes `--projects '[]'` expressible, so this is now a paper cut rather than a blocker. No overlay carried.

**Retire when:** the back-end Zod validator makes `projects`/`description` optional on metric-group create (growthbook repo fix).

Start with the postMetricGroup spec in the spec/back-end area and compare its required projects and description fields with sibling create specs. Then inspect the growthbook repo's back-end Zod validator; done means metric-group creation accepts omitted projects and description while --projects '[]' remains expressible.

Written by the indexing model from the issue text.