hooks/hooks.json already ships only schema-valid keys with metadata in the
hooks.metadata.json sidecar. Add scripts/ci/check-hooks-schema-keys.js, a
strict allowlist check that fails when hooks/hooks.json or
hooks/codex-hooks.json carry any key outside their loader's documented set,
wire it into the npm test chain, and cover it with fixture tests.
Refs #3138, #3114
