Add git remote-helper transport (HelperConn)

Implement a gitproto.Conn backed by a git-remote-<scheme> binary, bridging git-sync's smart transport onto the helper's stateless-connect capability. This lets git-sync drive URL schemes it has no native transport for (e.g. entire://) by delegating auth and network I/O to the helper while still running the wire protocol itself.

Each Conn operation spawns its own helper process: the helper services one stateless-connect session per process, and its receive-pack path reads the pushed pack until stdin EOF, so a fresh process per RPC is the simplest correct model and naturally supports batched (multi-request) pushes. The v2 upload-pack response is framed with a trailing response-end (0002) packet, which is stripped so the consumer sees exactly the byte stream it would over HTTP; receive-pack responses run to EOF. The helper strips the smart-HTTP "# service=" banner, so advertisements match the bannerless SSH shape that the existing decoders already accept.

Tests drive a fake helper via the os/exec re-exec pattern, covering the v2 advertisement, response-end stripping, receive-pack read-to-EOF, and the fallback path, plus unit tests for the pkt-line framing helpers.