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/playbooks/login.md

@@ -0,0 +1,14 @@
+# Playbook: Login / Auth
+
+## Steps
+
+1. Cite ADR-0004
+2. Define Visitor vs User in API behavior (`.ai/domain.md`)
+3. Issue Bearer token; document in OpenAPI
+4. Middleware extracts `user_id`
+5. Persist session/user as designed (Postgres MVP; Redis deferred)
+6. H5: store token via adapter (`localStorage` key `yxg_token`)
+7. SDK `getToken` wired
+8. Tests: invalid token / expired / ownership
+9. Security review checklist
+10. Never accept `X-User-Id` alone as auth