coreapi: refresh spec post-1820/ENT-741 — drop StatusCode wrappers, rewire mirror remove

entiredb#1820 (accurate success codes) and the ENT-741 mirror refactor are now on main, so refresh the spec and simplify against it:

• normalize.go: the success-code bug is fixed upstream (creates declare 200, deletes 204), so stop collapsing success to a "2XX" range. Keep the real success code and fold only the explicit 4xx/5xx into a single "default" (all reference one ErrorModel) — still convenient errors, but ogen now returns the success type directly with no *…StatusCode wrapper. collapseTypeUnions stays: #1 (nullable-array shorthand) is still unfixed.
• Drop the .Response unwraps across org/project/repo/grant/repo_mirror now that methods return (*T, error); DELETEs return plain error.
• mirror remove: the by-mirror lookup endpoint is gone; call the new delete-by-coords route directly (DELETE /mirrors?provider&owner&repo& clusterHost). 404 is now a real, surfaced error, not idempotent success.
• UPSTREAM.md: reframe #2 (success-code bug fixed; the error-fold that remains is a deliberate ergonomics choice retired by a shared upstream "default"); remove #3 (resolved by delete-by-coords).

Net ~2.4k fewer generated lines. fmt/lint/test:ci green.