Close NNTP connections left idle for NNTP_IDLE_CLOSE_SECONDS

Eweka drops quiet connections, so the next command on one failed
("Spot inspection failed: NNTP connection closed"), and idle connections
held account slots that downloads could use. The coordinator pool now
closes free connections (and the plain one) on its sync once idle for
60 s; dry prober lanes, the spotter between rounds and the spot inspector
with nothing to inspect close theirs too, reconnecting when work comes.

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

REBUILD.md

@@ -344,3 +344,10 @@ g. The account's NNTP connections shared by priority (`nntp_shares`): the
    fixed split (38 + 8), under which the prober stayed at 8 while history
    was paused and the coordinator's connections idle (LOG.md 2026-10-18
    (75)). **Accepted** 2026-10-18.
+h. Idle NNTP connections closed: one with nothing to do for
+   `NNTP_IDLE_CLOSE_SECONDS` (60) is closed and opened again with work
+   (the coordinator's pool on its next sync, the plain one too; a dry
+   prober lane; the spotter between rounds; the spot inspector with no
+   spots). Eweka dropped the quiet ones and the next command failed
+   ("Spot inspection failed: NNTP connection closed"), and idle ones took
+   the account's connections from downloads. **Accepted** 2026-10-22.