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/playbooks/add-api.md

@@ -0,0 +1,19 @@
+# Playbook: Add API
+
+Use when: “新增xxx接口”
+
+## Steps
+
+1. **OpenAPI** — add path to `proto/openapi.yaml`
+2. **Migration** — if new/changed table (`apps/api/migrations/`)
+3. **Model** — `internal/model`
+4. **Repository** — SQL only
+5. **Service** — business + authz
+6. **Handler** — bind + envelope; interface at handler
+7. **Wire** — register route in `cmd/server` or router file
+8. **Test** — at least service/engine unit test if logic-heavy
+9. **SDK/types** — update `packages/sdk` + `packages/types` if clients need it
+10. **Commands** — `go test ./...`, hit with curl
+11. **Review + DoD + checklist/feature.md**
+
+Follow patterns: `handler.md` `service.md` `repository.md` `pagination.md`.