gateway: hot-reload agents.conf at runtime (#869)

Replace eval-time per-agent nginx location baking with a runtime
include file. c0re writes /var/lib/hyperhive/agents.conf (nginx
location blocks, UDS or TCP per agent) on every topology change and
on the 10s marker poll. The gateway container bind-mounts
/var/lib/hyperhive/ at /run/hive-state/ and nginx includes
/run/hive-state/agents.conf. A systemd path unit inside the container
watches the file for changes and fires `nginx -s reload` on each
atomic rename from c0re — no nixos-rebuild switch needed when agents
start, stop, or flip useUnixSocket.

  - new hive-c0re/src/gateway_nginx.rs: write() + render()
  - lib.rs + meta.rs + agent_sockets::spawn_poll: hook in write()
  - hive-gateway.nix: drop agentPortsTable/agentSocketsTable/
    agentUpstreamFor/lib.mapAttrs', add /run/hive-state bind-mount,
    include directive, systemd path unit + reload service, tmpfiles
    for /var/lib/hyperhive + agents.conf seed
  - docs/gateway.md: update vhost table + Per-agent UDS section

hive-c0re/src/lib.rs

@@ -16,6 +16,7 @@ pub mod actions;
 pub mod agent_ports;
 pub mod agent_server;
 pub mod agent_sockets;
+pub mod gateway_nginx;
 pub mod approvals;
 pub mod auto_update;
 pub mod broker;