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/workflow.md

@@ -0,0 +1,33 @@
+# Workflow — Default Development Loop
+
+```
+Receive task
+    ↓
+Read AI Contract + constitution + architecture + domain
+    ↓
+Architecture Check (ADR + file-map + layers)
+    ↓
+Choose playbook (add-api / new-page / new-table / …)
+    ↓
+Coding (follow patterns + examples)
+    ↓
+Commands (build / test / health)
+    ↓
+Review (.ai/review.md)
+    ↓
+DoD + checklist
+    ↓
+Commit (Conventional Commits, one concern)
+```
+
+## Architecture Check questions
+
+1. Does this change reverse an Accepted ADR? If yes → ASK / new ADR.
+2. Correct layer? Handler / Service / Repository?
+3. Domain words match `.ai/domain.md`?
+4. API envelope still `{code,message,data}`?
+
+## Stop conditions
+
+- Unclear requirement → ASK FIRST
+- Need GraphQL / new DB / new auth scheme → write or cite ADR before coding