[Feature] Export (backup) chat history in human-readable format for archive

Relying on Keybase chat history and attachments for important reference points, notes, facts or events means that backups need to be easily provided - e.g. a human-readable chat history dump with the attachments fetched and stored offline in the archive.

The sender, the date/time the message was created/sent, the content (text and/or image etc), _potentially the time it was edited (if any)_

This can be in any convenient format e.g. HTML, or Richtext, provided it can be sent onwards easily e.g. for audits, accounts, legal, or just to avoid catastrophic loss of important chat & file data.

The JSON format is already available via `keybase chat api` on the command line, but the resulting JSON could easily be converted to human-readable format, and **especially needs to have the attachments fetched and embedded/stored locally.**

Start with the existing `keybase chat api` command and its JSON output. Determine how a human-readable format such as HTML or Richtext should represent sender, timestamps, content, edits, and attachments, then define how attachments are fetched and stored locally. Done means an archive can be shared or inspected without relying on Keybase chat history or remote attachments.

Written by the indexing model from the issue text.