Parse and validate LLM, Slack, and GitHub adapter responses at the door
with zod, so downstream code consumes typed values instead of narrowing
`unknown` with `as`.

- github: lenient looseObject schemas for GraphQL nodes (graphql-schema.ts);
  ghGraphQL/ghRest take a schema and return Result<z.infer<S>>; normalize,
  discover-links, adapter, and auth consume typed nodes — drops the
  Record<string, unknown> threading and the boundary casts.
- slack: schemas for Web API replies in adapter.ts + identity.ts.
- llm: workersAiResponseSchema in extractText.

Parse failure -> Result.Err (or undefined where a Platform port demands it).
Schemas declare only consumed fields + passthrough, tolerant of upstream
additions.