Fix Poke webhook: verify success field, point key setup to Kitchen

- forward_to_poke now checks the response's success field instead of
  trusting the HTTP status alone; Poke can return 200 with
  success: false on a soft failure, which was previously logged and
  treated as delivered.
- README/start.sh pointed users to Settings > Advanced for the API
  key, which issues a legacy pk_ key incompatible with the
  inbound/api-message endpoint this project uses. Now points to
  poke.com/kitchen -> API Keys for a V2 key.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

start.sh

@@ -183,7 +183,9 @@ PYEOF
     echo ""
   else
     echo "  ⚠  poke_api_key not set in config.yml."
-    echo "     Run 'npx poke login' then restart, or paste your key from poke.com/settings/advanced"
+    echo "     Run 'npx poke login' then restart, or paste a V2 key from poke.com/kitchen -> API Keys"
+    echo "     (Settings > Advanced and the Recipes page issue a different, incompatible key —"
+    echo "      the webhook_url above requires a V2 key created in Kitchen)"
     echo ""
     printf "  Poke API key (leave blank to set manually later): "
     read -r POKE_TOKEN_INPUT