chore: rename packages from @oasty/* to @openapi-spec/*

[dinwwwh]

Renames the npm scope from @oasty/* to @openapi-spec/* so the published packages match the middleapi/openapi-spec repository. The packages are now @openapi-spec/types and @openapi-spec/downgrader, and no reference to the old name remains anywhere in the repo.

Changes

• Package names, the downgrader's workspace dependency, and the lockfile entry use the new scope.
• Homepage and repository URLs point at middleapi/openapi-spec, matching the git remote.
• The oasty keyword is replaced by openapi-spec in both packages.
• READMEs and the sponsor sync script use the new name, so the scheduled sponsors sync no longer reintroduces "Oasty".

Testing

• Install with a frozen lockfile succeeds, confirming the lockfile edit is consistent.
• Lint, typecheck, and both package builds pass.
• All 327 tests pass.

Notes

• The root README heading still reads "OAS"; it never referenced the old name, so it was left alone.
• Sherif reports the root package.json is missing a private field. That is pre-existing on main and unrelated to this change.