feat(cli): /put and /get in the interactive chat

/put FILE sends one local file, relative to the current directory or
absolute, into the chat's folder, then posts the application's attachment
message naming it ({"text": "", "attachment": {filename, size, type}}), so
the page shows it as its paperclip's. /get NAME saves a file of that folder,
named from it, into the current directory, never over a file already there.
The chat's folder is the page's rule: the operator's choice while its root
is writable and present, else the first such root; a chat_directory_ack
moves it. A line above the one written in shows the transfer's progress.

Attachment messages, from the page or from /put, read as [file: name, size]
in the room and in `chat`, and as an `attachment` field with --json.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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