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/bugfix.md

@@ -0,0 +1,17 @@
+# Prompt: Bugfix
+
+Load first: `.ai/constitution.md`, `.ai/coding.md`, `.ai/security.md`, `.ai/review.md`.
+
+## Process
+
+1. Reproduce or state the failure mode clearly.
+2. Find root cause (file + function). Prefer evidence over guess.
+3. Fix the smallest correct change.
+4. Add a regression test when the bug is in scoring, auth, or payment.
+5. Verify build/health.
+6. Output Review block (at least Architecture, Security, Test, Scope).
+
+## Forbidden
+
+- Drive-by refactors unrelated to the bug.
+- Swallowing errors to “make it pass”.