transport: add SIXT rental agreement

TODO.md

@@ -17,6 +17,8 @@ Open items before and during the trip. Tick and date when done.
   See [`flights-ms.md`](bookings/transport/flights-ms.md).
 - [x] **MS: move today's SIXT pickup after the flight delay** — completed;
   booking `9739014160` now shows 2:00 PM PDT pickup and 2026-10-05 1:30 PM PDT return.
+- [ ] **MS: have SIXT add LK as a permitted driver in writing before LK drives.**
+  The active rental agreement currently lists MS only.
 - [ ] **LK: confirm Budget LAX pickup/return location and price** —
   itinerary `73544810702403`. Hertz cancellation refund also TBC.
 - [ ] **Brand Plaza Glendale** room, price and payment TBC.