pty/ctl exec fails the write when the shell cannot start; a directory removed under a shell leaves its name

`exec` on pty/ctl restarted a shell in a directory that was gone, the
write succeeding and the pane dead; and the pane was then renamed
`<dir> (deleted)` from /proc, so a restart failed even once the directory
was back. `exec` is now refused up front, ENOENT, where the directory is
gone; a shell the host cannot start fails the waiting write (late
failure, as a Save's); and a `(deleted)` cwd is no name to take.

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

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