useragent: identify git-sync in outbound User-Agent

Replace the bare go-git User-Agent on outbound requests with a git-sync-flavoured string. Git wire-protocol traffic (smart-HTTP info-refs and upload-pack/receive-pack RPCs, plus the protocol v2 "agent=" capability) sends "git-sync/<version> go-git/<v>" to preserve go-git attribution servers and operators key off; non-git HTTP (the GitHub repo metadata call in the bootstrap strategy) sends just "git-sync/<version>".

A new internal/useragent package centralises the format and holds a package-level Version var. The CLI wires this from versioninfo.Version at startup so --version and the User-Agent agree; SDK consumers can override it before issuing requests.