chore: update proto, scripts, and docs for CH migration

- Update hostagent proto: firecracker_version → vmm_version in metadata
- Regenerate hostagent.pb.go
- Update .env.example: WRENN_FIRECRACKER_BIN → WRENN_CH_BIN
- Update Makefile: remove --isnotfc from dev-envd target
- Update prepare-wrenn-user.sh: firecracker → cloud-hypervisor paths
  and capability assignments
- Update wrenn-init.sh: disable write_zeroes on rootfs for dm-snapshot
  compatibility with CH
- Update README.md and CLAUDE.md: Firecracker → Cloud Hypervisor
  throughout

proto/hostagent/gen/hostagent.pb.go

@@ -155,7 +155,7 @@ type CreateSandboxResponse struct {
 	Status    string                 `protobuf:"bytes,2,opt,name=status,proto3" json:"status,omitempty"`
 	HostIp    string                 `protobuf:"bytes,3,opt,name=host_ip,json=hostIp,proto3" json:"host_ip,omitempty"`
 	// Runtime metadata collected during sandbox creation (e.g. envd_version,
-	// kernel_version, firecracker_version, agent_version).
+	// kernel_version, vmm_version, agent_version).
 	Metadata      map[string]string `protobuf:"bytes,4,rep,name=metadata,proto3" json:"metadata,omitempty" protobuf_key:"bytes,1,opt,name=key" protobuf_val:"bytes,2,opt,name=value"`
 	unknownFields protoimpl.UnknownFields
 	sizeCache     protoimpl.SizeCache