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)

Makefile

@@ -84,12 +84,8 @@ migrate-reset:
 generate: proto sqlc
 
 proto:
-	protoc --go_out=. --go_opt=paths=source_relative \
-		--go-grpc_out=. --go-grpc_opt=paths=source_relative \
-		proto/hostagent/hostagent.proto
-	protoc --go_out=. --go_opt=paths=source_relative \
-		--go-grpc_out=. --go-grpc_opt=paths=source_relative \
-		proto/envd/process.proto proto/envd/filesystem.proto
+	cd proto/envd && buf generate
+	cd $(ENVD_DIR)/spec && buf generate
 
 sqlc:
 	@if command -v sqlc > /dev/null; then sqlc generate; \