Start a chat with any harness, not only an agent

New chat now shows two labelled groups, Harnesses and Agents. Pick any active harness, such as Generic, Base or one of your custom harnesses, to start a chat bound directly to it. Before, the picker listed agents plus a single hard-coded Platform Chat option, so chatting with a harness meant creating an otherwise empty agent.

The thread names the harness as its counterpart and appears in the Chats list like any other chat.

See Harnesses, Generic harness and #3262.