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>

.ai/definition-of-done.md

@@ -0,0 +1,17 @@
+# Definition of Done
+
+Code finished ≠ Done.
+
+A feature is Done only when applicable items PASS:
+
+- [ ] Code (compiles / typechecks)
+- [ ] API (handler + service + repository as needed)
+- [ ] Test (unit or listed smoke for critical paths)
+- [ ] Migration (if schema changed)
+- [ ] Docker / deploy notes (if new service or env)
+- [ ] Docs (OpenAPI / short PRD note if user-visible)
+- [ ] Review (`.ai/review.md` checklist output)
+- [ ] Build (`go test ./...` and/or `npm run build:h5` for touched side)
+- [ ] Health check still green (`/api/v1/healthz`)
+
+If an item is N/A, state why. Silent skip = not Done.