docs(rfc): record the schema-validation alternatives considered

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.

docs/rfc/implemented/feature/2026-07-05-dynamic-workflows.md

@@ -56,6 +56,9 @@ A `workflow` tool mirroring `dsh-tool-subagent`'s synchronous shape: start, awai
 - **Workflow-layer JSON parsing for `agent({schema})`**: duplicating a seam concern at one consumer while the seam's capability flag stayed dishonestly `false`.
 - **Meta as tool parameters instead of `export const meta`**: zero parsing, but scripts stop being self-contained artifacts and CC-authored scripts stop being drop-in.
 - **`SchemaSpec` as the outputSchema type**: the author-facing DSL cannot express what arrives as data and cannot be validated against without conversion loss.
+- **A schema-object library (zod, or the repo's schemastery) for the structured-output subset**: the schema is wire data — plain JSON that crosses the vm realm boundary in `agent({schema})` and lands verbatim in the forced tool's parameters — exactly where live schema objects cannot sit; consuming raw JSON Schema at runtime would need a third-party converter on top (zod core only emits JSON Schema, not the reverse), and it would put a second schema language beside schemastery's config role.
+- **ajv for value validation**: it validates FULL JSON Schema, so the subset gate — the module's actual point, since every accepted keyword must be one the harness enforces — would remain hand-written regardless; it compiles validators through `new Function`; and it would be dsh-tools' first runtime dependency, all to replace the ~70-line value walker while the path-qualified, every-violation error reporting stays custom either way.
+- **Provider JSON mode (`response_format: {type: json_object}`) instead of the forced capture tool**: the official API guarantees valid JSON, not schema-conforming JSON (no `json_schema` type; the docs' own guidance is to validate client-side, with the schema riding in the prompt), so both walkers survive untouched and only the capture-tool mechanics could go — at the cost of tools during a structured child's run (whether `response_format` composes with tool calling is undocumented), the in-turn validation retry (`ToolArgsError` keeps recovery inside the turn; a JSON-mode empty body — a documented failure mode — ends the turn, and the only recovery is the re-prompt loop this design rejects), and a new per-adapter `LlmCallConfig` surface. The accepted upgrade path is strict TOOL schemas (provider-side constrained decoding on tool parameters) when available: the same forced tool and subset gate, with the gate narrowed to the provider's strict subset.
 
 ## Consequences