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/ai-contract.md

@@ -0,0 +1,36 @@
+# AI Contract
+
+Highest operational contract for every agent session.
+
+## Before coding
+
+MUST read:
+
+1. `.ai/constitution.md`
+2. `.ai/architecture.md`
+3. `.ai/domain.md` (+ `domain/domain-map.md` when touching models/API)
+4. `.ai/coding.md`
+5. Task-relevant: `api.md` / `database.md` / `ui.md` / `security.md` / `product/feature-map.md`
+6. UI tasks: `design/design-system.md` + `design/component-catalog.md` + `design/platform/*`
+7. Relevant ADR under `.ai/adr/` if changing stack or style
+
+## After coding
+
+MUST:
+
+1. Run / follow `.ai/review.md` and print Review block
+2. Verify `.ai/definition-of-done.md`
+3. Use matching checklist under `.ai/checklists/`
+4. Prefer verifying with `.ai/commands.md` (build/test/health)
+
+## Never
+
+- Guess API shape
+- Guess DB schema
+- Guess product requirement
+- Invent endpoints, tables, or domain words not in `.ai/domain.md` / OpenAPI / task
+- Silently reverse an Accepted ADR
+
+## If unclear
+
+**ASK FIRST.** Do not invent. Do not “assume and continue”.