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).

serving/docker-compose.yml

@@ -104,6 +104,9 @@ services:
       # Host-generic remote audit collector name (baked into the image config so
       # every host offloads auditd here; resolved per-environment to this host).
       - --address=/${AUDIT_HOST}/${SERVER_IP}
+      # Fleet-manager AGENT plane (:9600) — baked into the image so enrolled hosts
+      # dial the control plane by name; resolved here like the other fleet names.
+      - --address=/${MGMT_HOST}/${SERVER_IP}
       # Kiosk is LOCAL-ONLY (loads boot.fleet.lab), so sinkhole Chromium's
       # call-home to 0.0.0.0 — telemetry can't leave the lab even if the browser
       # tries. Defense-in-depth behind the in-image Chromium policy/flags; in