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/cmd/example/main.go

@@ -0,0 +1,55 @@
+// Command example demonstrates basic usage of the quicproquo Go SDK.
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"quicproquo.dev/sdk/go/qpq"
+)
+
+func main() {
+	addr := "127.0.0.1:5001"
+	if len(os.Args) > 1 {
+		addr = os.Args[1]
+	}
+
+	ctx := context.Background()
+
+	client, err := qpq.Connect(ctx, qpq.Options{
+		Addr:               addr,
+		InsecureSkipVerify: true,
+	})
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "connect failed: %v\n", err)
+		os.Exit(1)
+	}
+	defer client.Close()
+
+	status, err := client.Health(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "health check failed: %v\n", err)
+		os.Exit(1)
+	}
+	fmt.Printf("Server health: %s\n", status)
+
+	fmt.Println("\nUsage:")
+	fmt.Println("  // Authenticate with a pre-existing session token")
+	fmt.Println("  client.SetSessionToken(token)")
+	fmt.Println()
+	fmt.Println("  // Resolve a user's identity key")
+	fmt.Println("  key, err := client.ResolveUser(ctx, \"alice\")")
+	fmt.Println()
+	fmt.Println("  // Create a DM channel")
+	fmt.Println("  chID, wasNew, err := client.CreateChannel(ctx, peerKey)")
+	fmt.Println()
+	fmt.Println("  // Send a message")
+	fmt.Println("  seq, err := client.Send(ctx, recipientKey, payload)")
+	fmt.Println()
+	fmt.Println("  // Receive messages")
+	fmt.Println("  msgs, err := client.Receive(ctx, recipientKey)")
+	fmt.Println()
+	fmt.Println("  // Long-poll for messages (5 second timeout)")
+	fmt.Println("  msgs, err := client.ReceiveWait(ctx, recipientKey, 5000)")
+}