Remove the IPC socket of each shell a test starts (#13443)

* Answer the Elsewhen panel over the shell's IPC socket

The Elsewhen panel arrived with a plain IpcHandler, so omarchy-shell
reached it only through the slower qs ipc fallback, and the test that
every first-party handler registers as ShellIpc failed on quattro.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Remove the IPC socket of each shell a test starts

The runtime smoke and screenshot sanity tests start the real shell from a
temporary copy and kill it, which leaves its IPC socket behind in
XDG_RUNTIME_DIR. A session that runs the suite a few times collected
dozens of them.

base-test.sh gains shell_ipc_socket, which derives a shell's socket as
omarchy-shell does, and both tests remove theirs in cleanup. The socket
test derives its fixture's socket through it too, so the helper cannot
drift from omarchy-shell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

shell/plugins/panels/elsewhen/Panel.qml

@@ -616,7 +616,7 @@ Panel {
     onTriggered: root.refresh()
   }
 
-  IpcHandler {
+  ShellIpc {
     target: root.ipcTarget
     function open(): void { root.open() }
     function close(): void { root.close() }