fix(host_agent): a stalled NFS mount no longer freezes the agent (#5760)

#5760 "One stalled NFS mount freezes the whole agent: statvfs has no
deadline, so the host goes silent while it still answers ping". The agent
read every mount inline in its only loop; a hard-mounted share whose server
stops blocks statvfs in the kernel with no timeout. Each mount is now read
in a daemon thread under one 5 s deadline. One that misses it is sent as
stalled, and isn't probed again while its last probe is still blocked. A
mount that errors (a stale handle) is left out instead of emptying the list.

Steward counts them as mounts_stalled (alertable: "an NFS share stopped"),
and skips the fullest-disk figure when every mount stalled.

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

docs/plugins/host-agent-design.md

@@ -116,6 +116,10 @@ Python 3.8+ stdlib only. `urllib.request` for the POST, plus `json`, `os`, `shut
 - **SIGTERM** (from systemd) → finish current poll cycle → flush buffer one last time, with a `stopping` event saying whether the host is going down too (`systemctl is-system-running` says `stopping`) → exit.
 - **Systemd restart policy** → `Restart=on-failure`, `RestartSec=10`.
 
+### Mounts that don't answer (1.14.0, #5760)
+
+Each mount is read in a daemon thread under one 5 s deadline. A hard-mounted NFS share whose server stops blocks `statvfs` in the kernel with no timeout; read inline, that froze the whole agent. A mount that misses the deadline is sent as `{"mount", "stalled": true}`, and isn't probed again while its last probe is still blocked. Steward counts them as `mounts_stalled`.
+
 ### What happened to the host (1.14.0, #5756)
 
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