agent: dial the manager by a fleet DNS name (manager.fleet.lab)

The agent's baked default pointed at the PUBLIC admin-UI name
(diskless-server.bitasylum.org), which resolves to the serving host's outbound IP
— unreachable from the fleet LAN. There's no split-horizon to "automate": the
manager agent plane is just another *.fleet.lab, served by the fleet dnsmasq like
boot/nvme/audit.

- serving/.env.example: MGMT_HOST=manager.fleet.lab.
- serving/docker-compose.yml: dnsmasq --address=/${MGMT_HOST}/${SERVER_IP}.
- files/agent/agent.env + main.go default: FLEET_AGENT_MANAGER=manager.fleet.lab:9600.
- server-pki.sh: AGENT_SANS default -> manager.fleet.lab (cert SAN matches the
  baked name, so no FLEET_AGENT_SERVER_NAME override).
- docs/TODO #14 gotcha #4 updated: fleet name, not the IP+server-name workaround.

The public diskless-server.bitasylum.org stays the ADMIN-UI name (Traefik/:443).

files/agent/main.go

@@ -198,7 +198,7 @@ func logDown(down *fleetpb.ManagerDown) {
 // ---- config ----------------------------------------------------------------
 
 func loadConfig() config {
-	manager := env("FLEET_AGENT_MANAGER", "diskless-server.bitasylum.org:9600")
+	manager := env("FLEET_AGENT_MANAGER", "manager.fleet.lab:9600")
 	cfg := config{
 		manager:    manager,
 		serverName: env("FLEET_AGENT_SERVER_NAME", hostOf(manager)),