Add script to get fresh set of shells.

README.md

@@ -498,6 +498,13 @@ a terminal leaf). A terminal that has since exited reopens as a fresh shell in
 the recorded directory, as it would without herdr-core. Shells in the core can
 also be reached from anywhere else with `hc ls` and `hc attach`.
 
+Since the shells outlive every restart, so does anything wrong with them — an
+environment variable they started with, say. `mise run fresh-shells` replaces
+them all while keeping the layout: it quits playpen (saving the session), ends
+every terminal playpen started in the core, and relaunches, so each pane comes
+back as a new shell in its last directory. It lists what is running and asks
+first, and it is safe to run from inside one of those panes.
+
 The file is `session.json`, and it lives in the **state** directory rather than
 beside `layouts.json` and `settings.json`: