### Added - For `derivepassphrase vault`, in the SSH agent client, introduce a new abstraction layer responsible for establishing the connection to the SSH agent: "SSH agent socket providers". Reframe the existing code for connecting to SSH agents on UNIX as a standard socket provider, and reserve a second standard socket provider, yet to be written, for The Annoying Operating System. (The system is based on Python package [entry points][importlib.metadata.entry_points], and is open for third-party developers to develop their own socket providers in case the standard socket providers are insufficient.) - `derivepassphrase vault --version` now defers testing for SSH agent support (the "master SSH key" feature) to the SSH agent client constructor, instead of examining the environment itself and drawing its own conclusions. - For developers: Feature flags and similar enum values are now self-testing, i.e., they include [their own code for testing whether they are active/enabled/supported][derivepassphrase._types.FeatureTestEnum].