The #192 review discussion, preserved where design rationale lives: a
schema-object library (zod/schemastery) cannot sit at a wire-data boundary;
ajv replaces only the value walker while the subset gate — the module's
point — stays hand-written; provider JSON mode guarantees valid JSON, not
schema-conforming JSON, and would trade away mid-run tools and in-turn
validation retry for it. Strict tool schemas are named as the accepted
upgrade path when the provider ships them.