v0.4.0

relay: client-driven init phase for automatic commit negotiation

The VS Code extension now writes the client's commit hash to a .commit
sidecar file when connecting. The host relay waits for this file (with
the socket listener already up so the session is discoverable), then
sends a FrameInit (0x04) frame to the remote side. The remote provisions
the exact matching VS Code Server version and acknowledges before any
connections are accepted.

This eliminates version mismatch errors in stdio relay mode without
requiring any flags — the commit is negotiated automatically.

Also adds darwin-arm64 (Apple Silicon) to the CI build matrix.