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)

images/envd.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Wrenn envd guest agent
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/envd
+Restart=on-failure
+RestartSec=1
+
+[Install]
+WantedBy=multi-user.target