Move A2UI out of workspace-scoped feature flags and make it a global desktop feature flag. Removed workspace defaultFeatureFlags handling and related schema/typing, and added a2ui to desktop feature flag definitions, defaults and schemas. Updated the Feature Flags UI to a single global section and wire toggling A2UI to push a cowork/session/defaults/apply control event to active workspaces and reapply workspace defaults to active threads. Adjusted store, control socket, workspace defaults, web adapter, tests and docs to reflect the new behavior, and exposed shared feature-flag helpers from the desktop feature flag module.
