issue and PR history import from GitHub #17

repo import is deliberately git-data-only, so migrants abandon years of discussion. Add an importer for GitHub issues and pull requests: issues with comments/labels/state mapped directly; PRs land as closed/merged MRs with their discussion as comments (head refs where still fetchable). Attribute originals in the body (author + date header) since foreign users have no local account; token via stdin like repo import. One-shot, resumable per repo.