correct Buffer adapter GraphQL schema + infer plan tier

Follow-up fix to Phase 1a Social Publishing (#549). Three bugs caught while smoke-testing /integrations/buffer/refresh/ against Buffer's real GraphQL API on staging:

1. Organization has no plan field in Buffer's schema. Our validate_token_and_fetch_meta queried plan { tier } which fails with GRAPHQL_VALIDATION_FAILED. Plan tier isn't exposed at all via GraphQL.
2. Organization has no nested channels field. Channels are queried via a separate top-level channels(input: {organizationId: X}) query.