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/prompts/new-feature.md

@@ -0,0 +1,18 @@
+# Prompt: New Feature
+
+Load first: `.ai/constitution.md`, `.ai/architecture.md`, `.ai/coding.md`, `.ai/api.md`, `.ai/review.md`, `.ai/definition-of-done.md`.
+
+## Process
+
+1. Restate the feature in one sentence and list files you will touch.
+2. Follow feature flow: API → Service → Repository → Migration → SDK/types → UI → Test → Docs.
+3. Implement the smallest vertical slice that works end-to-end.
+4. Run builds/tests for touched sides.
+5. Output `.ai/review.md` Review block.
+6. Confirm Definition of Done.
+
+## Forbidden
+
+- Inventing APIs not in the task / OpenAPI.
+- Editing legacy root HTML unless migration is the task.
+- Shipping UI-only unlock/payment without server checks.