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/coding.md

@@ -0,0 +1,37 @@
+# Coding — Golden Rules
+
+## Size
+
+- One responsibility per file.
+- One responsibility per function.
+- No function longer than **50** lines.
+- No file longer than **400** lines (hard preference; split earlier if possible).
+- No circular dependency.
+
+## Go
+
+- Business packages by domain: `auth`, `profile`, `report`, `order`, `payment` — not `utils`.
+- Interface defined at **caller** (usually handler), implemented by service.
+- First parameter of I/O methods: `context.Context`.
+- Errors: `return fmt.Errorf("create user: %w", err)`.
+- Never `panic` in request path.
+- Never use package-level mutable globals for request state.
+- Export comments required on public funcs (what / in / out / side effects).
+
+## TypeScript / Vue
+
+- `strict: true`. No casual `any`.
+- Components: `PascalCase.vue` (`UserCard.vue`).
+- Pages under `src/pages/`, composables under `src/hooks/`.
+- No direct `fetch` in pages — use SDK.
+- Shared domain types go to `packages/types`.
+
+## Naming
+
+- Consistent domain words: profile, report, scale, membership, order.
+- Prefer boring clear names over clever short names.
+
+## Comments
+
+- Public APIs: always document.
+- Do not narrate obvious code. Document intent, invariants, side effects.