v0.3: Add file attachments to user messages

Files upload to the Anthropic Files API (ManagedAgentsClient::uploadFile) and
ride the user.message event as image/document content blocks built from the
new Attachment value object - the model reads them directly from its context,
no filesystem tools needed. The runner accepts the attachments as a fourth
run() argument and replays them on the stale-session retry; MessageDispatcher
carries their metadata through the queue. Covered by a runner test; the
Guzzle example implements the multipart upload.