feat: Sprint 7 — Go SDK with QUIC transport and Cap'n Proto RPC

First non-Rust client SDK for quicproquo ecosystem.

- Cap'n Proto codegen: generated 6487-line Go types from node.capnp
  with all 24 RPC methods (NodeService, Auth, Envelope)
- QUIC transport: quic-go + TLS 1.3, ALPN "capnp", single bidi stream,
  300s idle timeout, InsecureSkipVerify for dev, custom CA cert support
- High-level qpq package: Connect, Health, ResolveUser, CreateChannel,
  Send/SendWithTTL, Receive/ReceiveWait, DeleteAccount, OPAQUE wrappers
- Auth management: session token storage, version/token/deviceID on all RPCs
- Example program and README with API reference

All tests pass: go test ./..., go vet, go build

sdks/go/go.mod

@@ -0,0 +1,13 @@
+module quicproquo.dev/sdk/go
+
+go 1.25.7
+
+require (
+	capnproto.org/go/capnp/v3 v3.1.0-alpha.2 // indirect
+	github.com/colega/zeropool v0.0.0-20230505084239-6fb4a4f75381 // indirect
+	github.com/quic-go/quic-go v0.59.0 // indirect
+	golang.org/x/crypto v0.41.0 // indirect
+	golang.org/x/net v0.43.0 // indirect
+	golang.org/x/sync v0.16.0 // indirect
+	golang.org/x/sys v0.35.0 // indirect
+)