coreapi: re-refresh spec — creates return 201, retire collapseTypeUnions

Safety check against fresh entiredb main caught a contract drift since the last refresh: POST creates now declare 201 Created (verified in main: orgs/projects/repos/mirrors all DefaultStatus=StatusCreated), and the nullable-array shorthand is fixed ("type": "array", no null member).

Our committed spec still said 200 for creates, so the generated decoder routed the server's 201 to the error path — every create command would fail at runtime with a decode error. Re-fetch the spec (now byte-identical to live) and regenerate: the decoder gains case 201 and creates succeed.

With nullable arrays fixed upstream, drop the collapseTypeUnions transform (and nonNullType) entirely and close that UPSTREAM.md item — only the error-fold ergonomics transform remains. Generated Go types are unchanged (arrays were already []T); just the success-code decode path moves 200→201.

fmt/lint/test:ci green; committed spec == live.