chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -4,3 +4,23 @@ __pycache__/
 *.log
 .env
 .DS_Store
+
+# Node
+node_modules/
+dist/
+*.local
+
+# Go
+apps/api/bin/
+*.exe
+
+# IDE
+.idea/
+.vscode/
+
+# Data / local db
+*.db
+.scale_results.db
+
+# OS
+Thumbs.db