Get real docstrings out of zod schema .meta() descriptions

The agent-next processor contract (and, as the style spreads, every processor contract) documents every property with zod 4 .meta({ description }) instead of JSDoc comments. That is the right single source of truth — the descriptions ride the schema into runtime introspection — but today nothing turns them back into DEVELOPER-facing docstrings:

• z.infer/z.output types carry no JSDoc, so editor hover on state.config.llmRequestDebounceMs shows a bare number — the description is invisible exactly where an implementer reads it.
• The generated itx-api surface and the event docs site each have their own doc extraction; schema meta is not wired into either.

Look into:

1. A codegen step that emits .d.ts (or typed wrapper) declarations with the meta descriptions as JSDoc, so hover docs work — possibly as part of the existing itx-api generator.
2. Wiring meta descriptions into the events.iterate.com docs rendering for contract-owned events (payload property tables, not just the event-level description).
3. Whether TypeBox (or another schema library with first-class static-type docstrings) would serve better than zod for CONTRACT schemas specifically — weigh against how deep zod is in the platform (validation, defaults, prefault semantics, discriminated unions, the processor-contract machinery).

Origin: Jonas review of PR #2154 (2026-07-20) — "use zod4's zod.meta. also add a task in tasks/ to note that we need to look into getting correct docstrings out of these zod schemas in the future (or use typebox or sth)".