Add host-mode heartbeat: vetting-agent host + last-seen badge

vetting-agent gains a `host` subcommand that runs as a systemd service
installed by the quick-register one-liner, POSTing every 30s to
/api/v1/hosts/{mac}/heartbeat so the dashboard tile shows "online" or
"Nm ago" without waiting on WoL. Ships dormant client code for the
Phase 2 reboot_for_vetting command so the server can flip it on later
without a binary redeploy.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

deploy/vetting.production.yaml

@@ -39,6 +39,11 @@ pxe:
   tftp_root: "/var/lib/vetting/tftp"     # holds ipxe.efi + undionly.kpxe
   live_dir: "/var/lib/vetting/live"      # holds vmlinuz + initrd.img; served at /live/*
 
+agent:
+  # Directory holding vetting-agent-linux-amd64, served at
+  # /assets/vetting-agent-linux-amd64. install.sh drops the binary here.
+  asset_dir: "/var/lib/vetting/assets"
+
 # Notifications fire on StageFailed, SpecMismatch, HoldingOpened,
 # RunCompleted. Declare one or more notifiers and route each event
 # kind (and optionally severity) to a notifier by name. Delivery is