feat(#2363): full-DHCP for all agents — drop static agent_network_ip

All agent containers now receive their bridge IP dynamically via DHCP
from the dnsmasq pool instead of a hash-derived static address:

- nix/templates/harness-base.nix: networking.useDHCP = true
- nix/modules/hive-gateway.nix: expand DHCP pool to full usable range
  (.2 to .254 on /24) — was last-14-IPs-only
- hive-sh4re/src/priv_proto.rs: remove agent_ip from NetworkIsolation
- hive-c0re/src/lifecycle/mod.rs: drop agent_network_ip + DHCP_POOL_SIZE
- hive-c0re/src/lifecycle/host_config.rs: remove agent_network_ip call
- hive-priv/src/main.rs: LOCAL_ADDRESS= empty (DHCP assigns IP);
  HOST_ADDRESS still set so nixos-container installs default route
  before the DHCP lease arrives
- nix/dhcp-pool-size: deleted (no longer needed)

The nix/dhcp-pool-size single-source-of-truth file and all associated
Rust/Nix dual-constant plumbing are gone — there is no static map.
bridge_gateway_ip() is retained (still needed for HOST_ADDRESS).

Closes #2363

nix/templates/harness-base.nix

@@ -1695,6 +1695,10 @@ in
     # disabling dhcpcd itself, so the veth still gets its address); then
     # the hyperhive-isolated-dns oneshot owns resolv.conf. (Same "take
     # resolvconf out of the loop" approach the matrix container uses.)
+    # All agent containers receive their bridge IP via DHCP from the hive
+    # dnsmasq pool (see hive-gateway.nix).  useDHCP runs dhcpcd on every
+    # interface (just eth0 in practice — the nspawn bridge veth).
+    networking.useDHCP = true;
     networking.resolvconf.enable = false;
     networking.dhcpcd.extraConfig = "nohook resolv.conf";