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>

test/shell.d/runtime-smoke-test.sh

@@ -12,6 +12,7 @@ cleanup() {
     kill "$QS_PID" 2>/dev/null || true
     wait "$QS_PID" 2>/dev/null || true
   fi
+  [[ -n ${test_root:-} ]] && rm -f "$(shell_ipc_socket "$test_root")"
   [[ -n $TMPDIR && -d $TMPDIR ]] && rm -rf "$TMPDIR"
   return 0
 }