feat(ai-chat reference): db:reset:chats helper for clean smoke-test slates

The reference's Chat / ChatSession Postgres tables are shared between local and test cloud targets. A row created with one webapp's PAT and lastEventId is poison if you switch the .env to the other target and reuse the same chatId — the transport gets a 401 or resumes from a sequence that doesn't exist on the other backend.

Adds:
- prisma/reset-chats.sql: TRUNCATE Chat, ChatSession (User survives — it's upserted by onPreload/onChatStart anyway).
- package.json db:reset:chats script wrapping prisma db execute --file.

Run `pnpm run db:reset:chats` between target switches and at the top of every smoke test. Codified in the ai-chat-e2e skill as a required prereq.

references/ai-chat/package.json

@@ -10,6 +10,7 @@
     "db:migrate": "prisma migrate dev",
     "db:push": "prisma db push",
     "db:generate": "prisma generate",
+    "db:reset:chats": "prisma db execute --file prisma/reset-chats.sql",
     "test": "vitest"
   },
   "dependencies": {

references/ai-chat/prisma/reset-chats.sql

@@ -0,0 +1,7 @@
+-- Wipe customer-side chat state for a fresh smoke-test slate.
+-- Run via `pnpm run db:reset:chats`.
+-- Leaves User rows intact (they're upserted by onPreload/onChatStart),
+-- but clears every Chat + ChatSession so a chatId from one target
+-- (test cloud / local) can't carry stale session/PAT/lastEventId state
+-- into the other.
+TRUNCATE "Chat", "ChatSession";