Support git remote-helper schemes (entire://) as a transport

[Soph]

What

Adds a git remote-helper transport to git-sync so it can drive URL schemes it has no native transport for. When the scheme isn't one git-sync handles natively, it looks for a git-remote-<scheme> binary on PATH (the same convention git uses) and bridges its stateless-connect capability — delegating auth and network I/O to the helper while git-sync still runs the wire protocol.

The motivating case is Entire's own entire:// URLs:

git-sync replicate --all-refs \
  https://github.com/source-org/source-repo.git \
  entire://cluster-host/et/project/repo

Previously this failed with unsupported protocol scheme "entire" because the URL fell through to Go's HTTP transport. entire:// is backed by the git-remote-entire helper (which git clone entire://… invokes), so git-sync now delegates to it the same way.

How

internal/gitproto/helper.go adds HelperConn, a gitproto.Conn backed by a helper process. Key protocol details (verified against git-remote-entire's source + live probing):

• Fetch (upload-pack) uses the v2 stateless framing: ack, advertisement, per-request POST with a trailing 0002 response-end packet (stripped so the consumer sees the same byte stream it would over HTTP).
• Push (receive-pack) is a connect-style raw proxy that reads the pushed pack until stdin EOF and streams report-status back.
• A fresh helper process is spawned per RPC: the helper services one stateless-connect session per process, and receive-pack reads the pack to EOF — so per-RPC processes are the simplest correct model and naturally support batched (multi-request) pushes.
• The advertisement comes back banner-stripped (like SSH), which the existing DecodeV2Capabilities/decodeV1AdvRefs already accept.
• Before opening a session, dial probes the helper's capabilities and fails with a clear error if stateless-connect is absent (no connect fallback).

http/https/ssh are never routed to a helper — git-sync keeps its optimized native transports even though git ships git-remote-http(s).

Scope / limitations

• Requires a stateless-connect-capable helper (entire, modern git-remote-curl). Helpers offering only the legacy connect capability are rejected with a clear error, not supported.
• Like SSH, the helper bridge has no per-request byte accounting, so --stats omits helper-side throughput.

Testing

• New helper_test.go drives a fake helper via the os/exec re-exec pattern: v2 advertisement, response-end stripping, receive-pack read-to-EOF, fallback, missing-stateless-connect error, plus unit tests for the pkt-line framing helpers.
• Validated end-to-end against the live server: probe negotiates v2, and the real replicate from GitHub → entire:// pushed 55,008 refs (protocol=v2, relay-mode=bootstrap).
• go test ./..., go vet, and golangci-lint all clean.

🤖 Generated with Claude Code

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Note

Medium Risk
New transport path touches core sync connection setup and subprocess/protocol bridging; mistakes could break pushes or mis-handle schemes, but native http/https/ssh paths are unchanged and helpers are opt-in via PATH.

Overview
Adds a remote-helper transport so non-native URL schemes (e.g. entire://) work when git-remote-<scheme> is on PATH, matching how git resolves custom remotes.

newConn in the syncer now routes ssh and http/https to existing native transports; other schemes use HelperConn when a helper exists, otherwise behavior stays as before (unsupported scheme from HTTP). http/https are never delegated to git-remote-http(s).

HelperConn implements gitproto.Conn by spawning a helper per RPC, probing for stateless-connect (no legacy connect fallback), driving upload-pack/receive-pack over the helper bridge, stripping v2 0002 response-end framing, and passing the user URL verbatim to the helper for auth (e.g. Entire CLI / git-remote-entire).

Pkt-line handling gains readRawPktLine for verbatim on-wire frames used when reading helper advertisements and responses. README and usage docs describe helper schemes, entire:// examples, and that --stats omits helper-side byte counts (like SSH).

^{Reviewed by Cursor Bugbot for commit 4ff9494. Configure here.}