Feature flags move to Statsig

Feature flagging moves to Statsig, integrated through the native Statsig SDKs on each platform. No abstraction layer sits in between, and the earlier OpenFeature approach is superseded. A repository uses one provider at a time. New work starts on Statsig. A repository still on LaunchDarkly or OpenFeature migrates through a planned project that removes the old provider.

See Feature flags for the standard, and ADR-0005 for the decision and its alternatives.