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/database.md

@@ -0,0 +1,39 @@
+# Database — Golden Rules
+
+## Naming
+
+- `snake_case` only for tables and columns.
+- Good: `user_profile`, `payment_order`, `user_session`.
+- Forbidden prefixes: `tbl_`, `t_`.
+
+## Required columns on business tables
+
+```
+id
+created_at
+updated_at
+deleted_at
+```
+
+## Types
+
+- Never `varchar(5000)` as a habit.
+- Prefer bounded `varchar(n)`.
+- `TEXT` only when necessary (long content bodies).
+- Timestamps: `timestamptz`.
+
+## Keys & indexes
+
+- Every foreign key indexed.
+- Unique business keys enforced with UNIQUE.
+
+## Migrations
+
+- Schema change without migration = incomplete feature.
+- Migrations live in `apps/api/migrations/`.
+- Provide Up and Down when tool supports it.
+
+## Soft delete
+
+- Default: set `deleted_at`, do not hard delete user content in MVP.
+- Queries must filter `deleted_at IS NULL` unless explicitly including deleted.