Pre-existing CLI improvements ahead of the tarball-publish refactor:
- New top-level `ninja scope` command (create, list, set-default).
- `init` accepts no --scope: prompts from ListMyScopes or uses creds default.
- Plugin name prompted if not provided.
- `plugin bump <major|minor|patch>` writes the bumped version into plugin.mod.
- `plugin version` prints the current plugin.mod version.
- `login` prints a URL with ?user_code= so the link is one click.
- creds: HostCreds gains optional default_scope.
- plugin/version: ParseBaseSemver + BumpVersion helpers, with tests.
