fix: add missing env vars to turbo.json for kilo-docs build

Declare POSTHOG_API_KEY and FREE_TIER_AMOUNT in the
@kilocode/kilo-docs#build task so Vercel exposes them during
builds and Turborepo includes them in the cache hash.

turbo.json

@@ -15,6 +15,11 @@
     "@opencode-ai/app#test": {
       "dependsOn": ["^build"],
       "outputs": []
+    },
+    "@kilocode/kilo-docs#build": {
+      "dependsOn": ["^build"],
+      "outputs": ["dist/**"],
+      "env": ["POSTHOG_API_KEY", "FREE_TIER_AMOUNT"]
     }
   }
 }