Convert ENABLE_WIREGUARD (`pkg/wireguard/agent/cell.go` - WireGuard enabled) to load-time config

Convert this macro to use DECLARE_CONFIG or NODE_CONFIG for runtime configuration.

Current definition location: pkg/wireguard/agent/cell.go

 if a.Enabled() {
     out.NodeDefines = map[string]string{
         "ENABLE_WIREGUARD": "1",
     }
 }

BPF usage: Used in conditionals in:

• bpf/bpf_host.c - WireGuard tunnel device handling