docs(#2363): update network.md + hive-ci.nix for full-DHCP model

- docs/network.md: LOCAL_ADDRESS is now empty (not deterministic-IP);
  containers use dhcpcd + dnsmasq DHCP pool, not hash-derived static IPs
- nix/modules/hive-ci.nix: 'service containers' → 'all containers'
  in the dnsmasq DHCP pool comment (agents also use the same pool)

nix/modules/hive-ci.nix

@@ -410,9 +410,9 @@ in
           # affect traffic destined for the bridge IP itself.
           networking.nameservers = [ networkCfg.bridgeIp ];
           # Bridge-attached via privateNetwork=true + hostBridge. The
-          # gateway's dnsmasq now serves a DHCP pool for service containers
-          # (see dhcp-range in hive-gateway.nix). hive-ci gets its address
-          # from that pool; no static address needed.
+          # gateway's dnsmasq serves a DHCP pool covering all usable bridge
+          # addresses (see dhcp-range in hive-gateway.nix) — agents and
+          # service containers alike receive IPs dynamically.
           networking.interfaces.eth0.useDHCP = true;
 
           # nspawn containers can't create user-namespaces, so nix