Move brainstorm-server into skill directory per agentskills spec

Moves lib/brainstorm-server/ → skills/brainstorming/scripts/ so the
brainstorming skill uses relative paths (scripts/start-server.sh) instead
of ${CLAUDE_PLUGIN_ROOT}/lib/brainstorm-server/. This follows the
agentskills.io specification for portable, cross-platform skills.

Updates visual-companion.md references and test paths. All tests pass.

skills/brainstorming/scripts/stop-server.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# Stop the brainstorm server and clean up
+# Usage: stop-server.sh <screen_dir>
+#
+# Kills the server process. Only deletes session directory if it's
+# under /tmp (ephemeral). Persistent directories (.superpowers/) are
+# kept so mockups can be reviewed later.
+
+SCREEN_DIR="$1"
+
+if [[ -z "$SCREEN_DIR" ]]; then
+  echo '{"error": "Usage: stop-server.sh <screen_dir>"}'
+  exit 1
+fi
+
+PID_FILE="${SCREEN_DIR}/.server.pid"
+
+if [[ -f "$PID_FILE" ]]; then
+  pid=$(cat "$PID_FILE")
+  kill "$pid" 2>/dev/null
+  rm -f "$PID_FILE" "${SCREEN_DIR}/.server.log"
+
+  # Only delete ephemeral /tmp directories
+  if [[ "$SCREEN_DIR" == /tmp/* ]]; then
+    rm -rf "$SCREEN_DIR"
+  fi
+
+  echo '{"status": "stopped"}'
+else
+  echo '{"status": "not_running"}'
+fi