Add host agent with VM lifecycle, TAP networking, and envd client

Implements Phase 1: boot a Firecracker microVM, execute a command inside
it via envd, and get the output back. Uses raw Firecracker HTTP API via
Unix socket (not the Go SDK) for full control over the VM lifecycle.

- internal/vm: VM manager with create/pause/resume/destroy, Firecracker
  HTTP client, process launcher with unshare + ip netns exec isolation
- internal/network: per-sandbox network namespace with veth pair, TAP
  device, NAT rules, and IP forwarding
- internal/envdclient: Connect RPC client for envd process/filesystem
  services with health check retry
- cmd/host-agent: demo binary that boots a VM, runs "echo hello", prints
  output, and cleans up
- proto/envd: canonical proto files with buf + protoc-gen-connect-go
  code generation
- images/wrenn-init.sh: minimal PID 1 init script for guest VMs
- CLAUDE.md: updated architecture to reflect TAP networking (not vsock)
  and Firecracker HTTP API (not Go SDK)

internal/envdclient/health.go

@@ -0,0 +1,52 @@
+package envdclient
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// WaitUntilReady polls envd's health endpoint until it responds successfully
+// or the context is cancelled. It retries every retryInterval.
+func (c *Client) WaitUntilReady(ctx context.Context) error {
+	const retryInterval = 100 * time.Millisecond
+
+	slog.Info("waiting for envd to be ready", "url", c.healthURL)
+
+	ticker := time.NewTicker(retryInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return fmt.Errorf("envd not ready: %w", ctx.Err())
+		case <-ticker.C:
+			if err := c.healthCheck(ctx); err == nil {
+				slog.Info("envd is ready", "host", c.hostIP)
+				return nil
+			}
+		}
+	}
+}
+
+// healthCheck sends a single GET /health request to envd.
+func (c *Client) healthCheck(ctx context.Context) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.healthURL, nil)
+	if err != nil {
+		return err
+	}
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusNoContent {
+		return fmt.Errorf("health check returned %d", resp.StatusCode)
+	}
+
+	return nil
+}