feat(auth): add @ketopath/auth package wrapping Better Auth

- New workspace packages/auth exporting:
  - auth: Better Auth server instance using Prisma adapter on @ketopath/db
  - makeAuthClient: React client factory parameterised by baseURL
  - readAuthEnv: Zod-validated env reader (BETTER_AUTH_SECRET min 32 chars,
    BETTER_AUTH_URL, optional GOOGLE_CLIENT_ID/SECRET)
- emailAndPassword enabled with min 8 chars, requireEmailVerification: false
  for MVP (re-enable in V1, see ADR 0001)
- Google provider conditionally registered when both env vars are present —
  keeps the package usable before OAuth configuration is delivered
- 7-day sessions, 24h rolling refresh, cookie prefix 'ketopath'

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/auth/src/index.ts

@@ -0,0 +1,2 @@
+export { auth, type Auth } from './server.js';
+export { readAuthEnv, type AuthEnv } from './env.js';