Add --remote flag to select which git remote to resolve

[andrew]

Closes #42.

forge previously hardcoded origin as the remote to read when figuring out which forge to talk to. With multiple remotes pointing at different forges (a Gitea origin and a GitHub mirror, say) there was no way to operate on the second one without -R owner/repo.

The new persistent --remote flag picks which remote URL to parse. Defaults to origin so existing behaviour is unchanged.

Implementation uses a package-level setter on internal/resolve called from PersistentPreRun, rather than threading a third parameter through the 75 identical resolve.Repo(flagRepo, flagForgeType) call sites. The remote name is session-wide state so this fits.

The git remote get-url lookup and URL parse are extracted into resolveRemote() so they can be tested without the network-touching client construction. Tests use a real temp git repo with two remotes — t.Chdir plus git init plus git remote add — and verify that switching the remote name produces a different domain.

Error messages now name which remote failed: reading remote "mirror" (not in a git repo, or remote not configured; use -R or --remote).