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`
 ```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