fix: Windows brainstorm server lifecycle, restore execution choice

- Skip OWNER_PID monitoring on Windows/MSYS2 where the PID namespace is
  invisible to Node.js, preventing server self-termination after 60s (#770)
- Document run_in_background: true for Claude Code on Windows (#767)
- Restore user choice between subagent-driven and inline execution after
  plan writing; subagent-driven is recommended but no longer mandatory
- Add Windows lifecycle test script verified on Windows 11 VM
- Note #723 (stop-server.sh reliability) as already fixed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/brainstorming/scripts/start-server.sh

@@ -106,6 +106,12 @@ if [[ -z "$OWNER_PID" || "$OWNER_PID" == "1" ]]; then
   OWNER_PID="$PPID"
 fi
 
+# On Windows/MSYS2, the MSYS2 PID namespace is invisible to Node.js.
+# Skip owner-PID monitoring — the 30-minute idle timeout prevents orphans.
+case "${OSTYPE:-}" in
+  msys*|cygwin*|mingw*) OWNER_PID="" ;;
+esac
+
 # Foreground mode for environments that reap detached/background processes.
 if [[ "$FOREGROUND" == "true" ]]; then
   echo "$$" > "$PID_FILE"