coreapi commands: unit-test the pure helpers

The OpenAPI-first rewrite ported no tests from entiredb's command tree. Most of those tests covered functionality we dropped (data-plane content verbs, the mirror-use local-remote rewrite, handle resolution, the cross-juris 421 path, the cluster-positional parser), so they don't apply. But a few covered logic we carried over or wrote fresh and left untested — close that gap:

• parseGitHubURL: port entiredb's TestParseGitHubURL verbatim (the function was copied verbatim, so its test should come too).
• mirrorRow: clone-URL synthesis + private yes/no.
• clusterArg: explicit vs defaulted cluster host (and drop the unused idx parameter unparam flagged — the cluster is always args[1]).
• parseProjectOwnerType: org/account mapping + rejection.
• printTable / printFields: the plain (non-TTY) table and field layout.

Still uncovered by design: the HTTP-bound probe loop / 401 re-mint (integration-shaped, needs a fake server) — noted as a follow-up rather than faked here.