auths init: registry should be pushed to remote automatically

[bordumb]

Problem

auths init creates the identity and writes attestations to refs/auths/registry in ~/.auths/.git, but the user must manually:

1. git fetch ~/.auths refs/auths/registry:refs/auths/registry — pull registry into the project repo
2. git push origin refs/auths/registry — push to remote

This is non-obvious and undiscoverable. New users don't know the registry exists in ~/.auths, and the fetch-from-local-path pattern is uncommon.

Expected behavior

After auths init (when run inside a git repo):

• Automatically copy refs/auths/registry from ~/.auths into the current repo
• Prompt or auto-push to the remote

This is related to but distinct from #73 (pre-push hook for ongoing sync). This issue is about the initial setup experience.

Context

During dogfooding, auths init --force completed successfully but the widget showed errors because the registry was never pushed to the remote. Required manual git plumbing to fix.