zero changes: add push and pr subcommands (+ repo-info branch/commit/tag counts)

[Vasanthdev2004]

The zero changes suite can stage and commit, but there's no way to push the branch or open a PR without dropping to a separate git/gh invocation. Add:

• zero changes push — push the current branch to its tracking remote (fall back to origin); --force maps to --force-with-lease; --dry-run; with a detached-HEAD guard.
• zero changes pr — wrap gh pr create (--fill, or --title/--body, --draft), refusing to run interactively without --fill/--title.

Also add BranchCount / CommitCount / TagCount to zero repo-info, keeping it read-only.

Implemented in #391.