v0.2.0

Added

• Attachments by reference: an attachment with ref: (the host's own
  id for the file, a material id say) is stored in the session as its ref
  and metadata, never its bytes (no image part; the ref in the envelope and
  in the message's attachments; path is then optional). Each model
  call fetches a referenced picture (when the model takes pictures) or PDF
  (when it takes PDFs) through the new :resolve_attachment hook
  (fn attachment, ctx -> {:ok, {mime, bytes}} | {:error, reason} end or
  an MFA; ctx is %{session_id: id}), once per kind and ref, in the call's own
  process, not the loop's, and sends it where an inline one would go; with
  no hook, an error, a raise or a bad answer the model gets "(
  couldn't be loaded, so it isn't sent.)" instead. A tool can answer with
  pictures by reference, %{ref:, mime:, name:} in its :images, stored
  as the tool result's attachments (one answer's pictures are all inline
  or all by reference). Rho.Session.attachments/1 lists a user message's
  or tool result's files (kind, name, mimeType, and ref / path /
  about / pages when present) for the host's chat. Sessions with inline
  pictures are read as before, and pictures the model makes stay inline.
• Rho.Session.fetch/2: Rho.Session.messages/2 leaves placeholders for
  the files whose bytes aren't in the session (pictures by reference, PDFs)
  with fetch: false, to be fetched later in another process; without it
  (the default) it fetches them before returning, given session_id:.

Changed

• Attached PDFs read from their paths are read by the model call's
  process, not the loop's, and each attachment counts against the
  100-page / 20 MB budget (one attached twice used to go twice on one
  place in it). Rho.Loop.context/1 fetches in the caller's process.
• Rho.Loop.prompt/3, steer/3 and follow_up/3 raise ArgumentError
  in the caller for an attachment with neither :path nor :ref.