feat(host_agent): a host says what updates it has waiting and whether it needs a reboot (#5765)

Step 1 of milestone 514 "Updates and reboots on a cadence, carried out by
each host's agent…": #5765 "The agent reports pending updates and whether
a reboot is required".

- Agent 1.16.0: an hourly background check (apt-check, apt list, or dnf -C
  with updateinfo and needs-restarting), each command bounded at 120 s, no
  root and no network. Debian's reboot-required marker and its packages
  are read every sample.
- Ingest records updates_pending, updates_security and reboot_required for
  rules, and keeps the newest on the registration (host_agent_005_updates).
- The Agent card leads with "Reboot required since …", then the updates
  waiting or "Up to date", with when the check ran. list_agents carries
  them too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plugins/host-agent-design.md

@@ -124,6 +124,10 @@ Each mount is read in a daemon thread under one 5 s deadline. A hard-mounted NFS
 
 One Docker answer must arrive within 15 s and fit in 32 MiB, and one log answer within 5 s and 4 MiB. Past either limit the read is cut off, not waited on. A per-read timeout alone never ends a body that keeps trickling in. Container logs are read since each container's cursor, but only its newest 2,000 lines, all containers within 10 s. A container that writes more than that gets a `_steward` note saying older lines were skipped. A container whose answer is too big skips ahead to now. Before this, a 1.13.0 agent re-read a growing backlog every interval and went silent for a day at 2.1 GB. Each sample also carries the agent's own `agent` footprint (`rss_bytes`, `cpu_pct`), which Steward records as `agent_rss_bytes` and `agent_cpu_pct`.
 
+### What the host has waiting (1.16.0, #5765)
+
+Once an hour, and when the agent starts, a background thread asks the package manager what it would upgrade, so a slow apt never holds up a sample. On apt hosts it uses `/usr/lib/update-notifier/apt-check` (pending;security), or `apt list --upgradable` when that's missing. On dnf hosts it uses `dnf -C check-update`, `dnf -C updateinfo list --security`, and `needs-restarting -r` for whether to reboot. None of this needs root or the network: the commands read lists the system's own timers already fetched. Each command has 120 s. The last answer rides every sample as `updates`. Every sample also reads Debian's `/var/run/reboot-required` (and `.pkgs`) as `reboot`. Steward records `updates_pending`, `updates_security` and `reboot_required` (1/0) for rules, and keeps the newest on the registration. The host page's Agent card leads with "Reboot required since …", then "N updates waiting (M security)" or "Up to date", with when the check ran. `state["updates_due"] = True` asks for a check on the next sample; a maintenance job uses it once it has updated the host.
+
 ### What happened to the host (1.14.0, #5756)
 
 A post may carry `events`, said once each, with the next post that gets through: