init: scaffold duplicate_chess project and design spec

Local browser sandbox for "duplicate chess" — a four-player coupled-board
chess variant invented by Andrew Freiberg. Scaffold per CREATE_PROJECT.md
plus the approved design spec from this session's brainstorming.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,7 @@
+node_modules/
+dist/
+.superpowers/
+.DS_Store
+*.local
+*.tsbuildinfo
+GITEA_API.md