OpenAPI root document: remaining wire gaps vs PostgREST v14.12

[milmazz]

Follow-ups discovered while closing #53 items 2–3 (all verified against v14.12 src/PostgREST/Response/OpenAPI.hs, none asserted by a conformance case):

• Root "/" path item. PostgREST emits a paths./ entry (GET, tags ["Introspection"], summary "OpenAPI description (this document)", produces [application/openapi+json, application/json]) — makeRootPathItem, OpenAPI.hs#L370-379. Bier omits it.
• RPC operation produces list. Every RPC operation carries produces: [application/json, application/vnd.pgrst.object+json;nulls=stripped, application/vnd.pgrst.object+json] — OpenAPI.hs#L360. Bier emits no produces on RPC operations.
• Default host/basePath without a proxy. postgrestSpec always receives scheme/host/port/basePath (from pickProxy falling back to server config) — OpenAPI.hs#L393+. Verify what a live 14.12 emits with no openapi-server-proxy-uri and align (Bier currently emits neither host nor schemes in that case).

Source: research for #53 (PR #83); see docs/superpowers/plans/2026-07-16-issue-53-openapi-followups.md on that branch.

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