A pty/ctl exec that cannot start its shell keeps the one running

Each host closed the running shell before it forked the new one, so an exec whose shell failed left the pane with none and later runs answered error shell gone. A shell not there is now refused up front, and every host starts the new shell first, replacing the old only once the close-on-exec pipe says it ran; a failure there is only said (restartFailed).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 4e3428d1..68cc38a9 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -958,7 +958,9 @@ its directory: one that is gone is refused before anything runs, `exec:
 there, not executable, a script whose interpreter is not there) fails the
 write with why -- `shell: shell not found`, or `shell: interpreter
 /no/such/interp not found` for a script whose `#!` names a program that is
-not there, ENOENT -- keeping the terminal; a `Tty` naming such a shell or
+not there, ENOENT -- keeping the terminal and its running shell: a shell
+not there is refused before anything runs, and the host starts the new one
+before the old goes, so one that cannot start leaves the old be; a `Tty` naming such a shell or
 script is refused before anything runs (`Tty: interpreter ... not found`,
 its `err` the only record), and one whose shell cannot start fails the
 same way and leaves no pane. The host knows before it answers: the child reports a failed exec