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/tech-stack.md

@@ -0,0 +1,42 @@
+# Tech Stack — frozen unless ADR says otherwise
+
+## Backend
+
+- Language: Go 1.22+
+- HTTP: gin
+- DB: PostgreSQL
+- Migrations: goose (or golang-migrate) under `apps/api/migrations/`
+- Log: structured (zap or slog wrapper). No `fmt.Println` in business paths.
+- Module path: `github.com/yuxingu/digital-psychology/apps/api`
+
+## User H5
+
+- Vue 3 + TypeScript (`strict: true`) + Vite
+- Router: vue-router
+- State: pinia
+- Package: `@yuxingu/user-h5`
+
+## Shared
+
+- npm workspaces at repo root
+- `@yuxingu/sdk` `createClient({ baseURL, adapters })`
+- TS camelCase in app; SDK converts to/from API snake_case
+
+## Mini program
+
+- Native WeChat mini-program structure
+- Same SDK via adapters (`wx.request`, storage)
+- No shared Vue SFC with H5
+
+## Deploy
+
+- Docker Compose for local Postgres: `deploy/docker-compose.yml`
+- API health: `/api/v1/healthz` (liveness). Add readiness when DB is wired.
+- Never deploy with image tag `latest` in prod.
+
+## Do not introduce without ADR
+
+- Another backend language
+- GraphQL / gRPC as primary public API (REST is primary)
+- ORM that hides SQL without team agreement
+- New state manager besides Pinia on H5