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/checklists/bugfix.md

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+# Checklist: Bugfix
+
+- [ ] Failure mode stated
+- [ ] Root cause located (file/func)
+- [ ] Minimal fix only
+- [ ] Regression test if scoring/auth/payment
+- [ ] No unrelated refactors
+- [ ] Build/tests green
+- [ ] Review: Architecture / Security / Test / Scope

.ai/checklists/feature.md

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+# Checklist: Feature
+
+- [ ] Architecture layers respected
+- [ ] Domain words match `.ai/domain.md`
+- [ ] API (+ OpenAPI if public)
+- [ ] Service
+- [ ] Repository (if persistence)
+- [ ] Migration (if schema)
+- [ ] SDK/types (if client needs)
+- [ ] UI (if user-facing)
+- [ ] Test or smoke steps
+- [ ] Docs touched if behavior user-visible
+- [ ] `go test` / `npm run build:h5` as applicable
+- [ ] Health still OK
+- [ ] Review block printed
+- [ ] DoD satisfied

.ai/checklists/refactor.md

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+# Checklist: Refactor
+
+- [ ] Goal / non-goals stated (behavior unchanged unless asked)
+- [ ] No ADR silently reversed
+- [ ] Layers still clean
+- [ ] File/function size improved toward limits
+- [ ] Tests still pass; added if extracting unscored logic
+- [ ] Review: Architecture / Readability / Test / Scope

.ai/checklists/release.md

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+# Checklist: Release
+
+- [ ] main green: `go test ./...`, `npm run build:h5`
+- [ ] Migrations applied plan documented
+- [ ] Health endpoint verified
+- [ ] No secrets in artifact
+- [ ] Image tag ≠ `latest` (prod)
+- [ ] OpenAPI matches deployed routes
+- [ ] Changelog / version tag if shipping
+- [ ] Rollback path known