docs: add TODO list and settle 2026 park entry rules

Collect the open action items scattered across booking records into
TODO.md, starting with the US$250 nonresident annual pass. Yosemite has
no timed-entry reservation in 2026, and per NPS one nonresident pass
admits the whole vehicle and waives the per-person surcharge at both
parks, so update the parks notes accordingly.

README.md

@@ -4,6 +4,7 @@ Group trip: SF → Yosemite → Sequoia → LA → Las Vegas → SF.
 
 ## Structure
 
+- [`TODO.md`](TODO.md) — open action items before and during the trip
 - [`itinerary.md`](itinerary.md) — bookings overview and timeline
 - [`days/`](days/) — one file per day: logistics, agreed plan, notes, addresses with map pins
 - [`expenses.md`](expenses.md) — shared-expense ledger and reconciliation notes