Export and portability

## Goal

Let users take Cortext content out of the system in useful formats.

## Scope

* Export pages from the shell.
* Export collection rows and field data from the shell.
* Support Markdown or HTML for pages.
* Support CSV or JSON for collections.
* Include media and attachment references where feasible.

Start by locating the shell entry points for exporting pages and collection data, then trace how Markdown, HTML, CSV, and JSON output could be represented. Check how media and attachment references are exposed. Done means pages and collection rows can be exported from the shell in the requested formats, with references included where feasible.

Written by the indexing model from the issue text.