feat: add grafana/loki/promtail vendor .debs

- toolchain/build-vendor.sh: packages the observability components that
  are not in Debian main — grafana (official OSS deb, vendored as-is),
  loki + promtail (static binaries from the Loki GitHub release, wrapped
  in minimal debs that install the overlays/server configs and systemd
  units from toolchain/vendor/). Version-pinned, overridable, and
  download failures skip with a warning (never break the OS build).
- configure-system.sh: auto-enables grafana-server/loki/promtail when
  their binaries land in the image.
- New `make vendor` target.

toolchain/vendor/promtail/promtail.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Arcline Promtail log shipper
+Documentation=https://grafana.com/docs/loki/latest/clients/promtail/
+After=network-online.target loki.service
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/promtail -config.file=/etc/promtail/promtail.yml
+Restart=on-failure
+RestartSec=5
+LimitNOFILE=65535
+
+[Install]
+WantedBy=multi-user.target