fix: setup accepts WG_PRIVATE_KEY/WG_PUBLIC_KEY env vars

Allows running make setup on hosts without wg binary or Python cryptography
library by passing pre-generated keys from another machine.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -44,6 +44,8 @@ setup:
 	CELL_DOMAIN=$(or $(CELL_DOMAIN),cell) \
 	VPN_ADDRESS=$(or $(VPN_ADDRESS),10.0.0.1/24) \
 	WG_PORT=$(or $(WG_PORT),51820) \
+	WG_PRIVATE_KEY="$(WG_PRIVATE_KEY)" \
+	WG_PUBLIC_KEY="$(WG_PUBLIC_KEY)" \
 	python3 scripts/setup_cell.py
 
 init-peers: