fix(artifacts): reject malformed remote paths

[jbetala7]

Summary

• reject artifact remote URLs that do not contain both owner and repo path segments
• keep valid GitHub/GitLab-style remotes working, including deeper self-hosted paths
• add regression coverage for host-only and owner-only malformed inputs

Fixes #1597.

Root cause

parse_url() stripped .git and only checked that host and owner_repo
were non-empty. For inputs like https://github.com or
ssh://git@github.com, parameter expansion assigned github.com to both the
host and path-like value, so the helper generated bogus remotes instead of
failing.

Testing

• bun test test/gstack-artifacts-url.test.ts
• bun test test/gstack-artifacts-init.test.ts -t "only gh authed"
• git diff --check