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>

steward/core/formatting.py

@@ -339,6 +339,7 @@ METRICS: dict[str, tuple[str, str]] = {
     "uptime_secs": ("Uptime", "time since the host booted"),
     "seconds_since_seen": ("Time since the agent reported", "over 3 minutes means the agent is offline"),
     "clock_off_secs": ("Clock off by", "how far the host's clock is from Steward's, either way"),
+    "mounts_stalled": ("Mounts not answering", "mounts the agent couldn't read in 5 s, e.g. an NFS share whose server stopped"),
 }
 
 # Where a rule's metric comes from, as the picker and the rules list say it.