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/adr/0005-use-redis.md

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+# ADR-0005 Redis — deferred
+
+## Status
+
+Deferred (not accepted for MVP)
+
+## Decision
+
+MVP does **not** require Redis. Use PostgreSQL + process memory only when safe.
+
+## Reason
+
+1. Reduce moving parts for first ship
+2. Membership/session can start in Postgres
+
+## When to revisit
+
+Rate limiting, hot session cache, or distributed locks → write ADR-0005b Accepted before adding Redis.
+
+## Never
+
+Never add Redis “just in case” without an Accepted ADR update.