`user-invocable: true` is not accepted by the current skill validator

Using Codex CLI:

The report only identifies the current skill validator and the user-invocable field; begin by finding the validator entry point and reproducing validation from the shown Codex CLI case. Confirm the expected schema for user-invocable: true and add or update a focused validator test so the field is accepted without regressing existing checks.

索引モデルが issue の本文から書いたものです。