docs(changelog): tell users about the stray-shell leak under an agent loop

The measurement is in the code; this is the half a user needs. A tab created
and closed again before the window has finished reconciling leaves its shell
running — 5 of 6 iterations, reproducibly, with no pause between the two
commands. A second's pause, or creating without closing, leaks none, so
somebody working in the window by hand will never meet it and an orchestration
loop will meet it on nearly every pass.

Under Known rather than Fixed, because it is not fixed. What is true is that
the strays are now findable three ways and that one command ends them all;
what is not is that anything ends them for you.

CHANGELOG.md

@@ -151,6 +151,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   loudly and never presses anything anywhere; to type a number as text, name
   the pane too (`tty7 send %5 83 --enter`) (#538).
 
+### Known
+
+- **A tab created and closed again before the window finishes reconciling
+  leaves its shell running.** With a tty7 window open, `tab new` immediately
+  followed by `tab close` strands a live shell on most iterations — measured
+  at 5 of 6, reproducibly. Pausing a second between them, or creating without
+  closing, strands none, so somebody driving the window by hand will not meet
+  it; an orchestration loop that opens a tab per task and closes it when the
+  task is done will, on nearly every pass. The strays are visible
+  (`tty7 pane ls --all`, the switcher, and `tty7 doctor`) and
+  `tty7 pane close --orphans` ends them, but nothing ends them for you yet.
+
 ### Fixed
 
 - **The daemon raises its own open-file limit, and says so when it still runs