v4.0.1

Changed

• The ask-agentdeck example is now jack. examples/ask-agentdeck/ is
  examples/jack/, its package is jack, and the agent is named Jack, so the docs
  assistant has one name rather than two. Run it with
  uvicorn jack.server:app --port 8100. Its ASK_AGENTDECK_* environment variables keep
  their names.
• The documentation landing page is rebuilt around one worked example. It builds Jack from
  an Agent to a live run instead of listing features, and every code block states the release
  it belongs to, so the examples that are not in 4.0.0 yet say so.