An image pasted into the chat lived only in the transcript. The system prompt of
every session names `session_files/<id>/` as the place its files live, so an agent
looking for "the attached images" went there, found nothing, and spent turns on
fruitless `find` calls before giving up.

Now the agent's non-hidden turn writes each pasted image into that directory as
`image_N.<ext>` — the extension sniffed from the bytes, the bytes stored as they
arrived, so a PNG keeps its alpha channel. Numbering runs across the session, and
a picture sent twice reuses the file already holding it.

The paths go into the hidden context copy of the message: images past the context
budget are not sent to the model at all, so the notice names them for `image_view`
instead. The line is written by the agent rather than the websocket handler, which
is the layer that knows the names the files actually took — including for a message
queued behind a running turn.

A save that fails is logged and skipped: it never costs the turn.