docs+test: the young-run window in SERVER-MODE/LANES; run.test plants its link in the real drop dir

run.test.js hardcoded /tmp/agenthub-drops-<uid>, which drops left for
<data dir>/drops long ago — it passed only while an old build's folder
survived in /tmp (gone after a reboot). Full suites: server 2887/2887,
client 1250/1250.

docs/SERVER-MODE.md

@@ -806,7 +806,10 @@ agenthub users provision alice      # the Linux account only (a user made before
   `~/.config/agenthub/settings.env`) but lacks the unit's
   `CAP_DAC_READ_SEARCH` cannot stat a member's 0700 `.agenthub/tmux`; from
   v2.7.3 on that `EACCES` counts as "maybe running" and the member's list is
-  asked through sudo, never read as "no panes".
+  asked through sudo, never read as "no panes". And a run younger than 20 s
+  is never ended by any server: only the server that started a pane writes it
+  into its own cached tmux list, so another one on the same database had not
+  listed it yet and ended a Codex pane seconds after its start.
 - **Update now** under the system unit restarts through
   `sudo -n systemctl restart --no-block agenthub.service` (from v2.7.3). An
   older build started a detached replacement instead, which kept the port