Context keys are now tuples instead of strings, inspired by
React Query's query keys. This prevents context collisions
when multiple instances of the same source type are registered.
Sources write to structured keys like
["aris.google-calendar", "nextEvent", { account: "work" }]
and consumers can query by prefix via context.find().
Co-authored-by: Ona <no-reply@ona.com>
Replace per-source services (LocationService, WeatherService,
TflService, FeedEngineService) with a single UserSessionManager
that owns all per-user state. Source creation is delegated to
thin FeedSourceProvider implementations per source type.
Co-authored-by: Ona <no-reply@ona.com>
