OpenAPI Onboard Plugin

Onboard an existing Backstage backend plugin to the repo's OpenAPI tooling. The core flow reverse-engineers an openapi.yaml from the plugin's existing Express router, generates a server stub via backstage-repo-tools, switches the router over to createOpenApiRouter, and verifies with yarn tsc + the package's tests.

The OpenAPI spec is the source of truth. The router is the starting point: we read it once to derive the spec, then the spec drives generation forward.

Up front: confirm scope with the user

Before writing anything, ask which of the optional steps the user wants. Default to off unless the user says yes.