fix: correct outputs path and add actual env var names

- Change outputs from dist/** to .next/** (Next.js default)
- Keep POSTHOG_API_KEY and FREE_TIER_AMOUNT to silence Vercel warnings
- Add NEXT_PUBLIC_POSTHOG_KEY and NEXT_PUBLIC_POSTHOG_HOST for
  correct Turborepo cache invalidation

turbo.json

@@ -18,8 +18,8 @@
     },
     "@kilocode/kilo-docs#build": {
       "dependsOn": ["^build"],
-      "outputs": ["dist/**"],
-      "env": ["POSTHOG_API_KEY", "FREE_TIER_AMOUNT"]
+      "outputs": [".next/**"],
+      "env": ["POSTHOG_API_KEY", "FREE_TIER_AMOUNT", "NEXT_PUBLIC_POSTHOG_KEY", "NEXT_PUBLIC_POSTHOG_HOST"]
     }
   }
 }