support description

[matt-beanland]

Extend Artefact so can have optional description. Add to Artefact.new.

Extend Artefact.Mermaid so when set is exported also for accessibility.

[matt-beanland]

%Artefact{
  id:          String.t(),          # generated UUID
  title:       String.t() | nil,    # human label
  description: String.t() | nil,    # human description — surfaced as Mermaid accDescr
  style:       atom() | nil,        # render style reference — not persisted in graph
  graph:       %Artefact.Graph{},   # the knowledge
  metadata:    map()                # open map — consumers add their own keys
}