Use this when the user asks to make or look up something in an AiAkiv app,
or to manage their AiAkiv account through the `console` app: teams and
invitations, org links, audit logs, save-target history, switching Main or
turning it off and on, project presets, and the cards they created. Do not
use it to save a memory; use `save_memory` for that. Cards are made with
`create_aiakiv_card`.

Writes when `action="create"`. Some apps publish the result to the open web:
the returned link is live the moment it comes back — unlisted, but reachable
by anyone holding the address — and that publication cannot be undone from
here. The content is written by the caller from memory the user can already
read; the server never calls a model.

Args:
    app: The app name. An unknown one returns `available_apps`.
    action: `"describe"` returns the app's field list and limits, which is
        how the exact input shape for an app is established; `"create"`
        writes. Apps may define further read actions, which `describe`
        lists.
    data: The app's payload — a JSON object, or a JSON string holding one.
        The string form survives transports that mangle long multi-line
        fields.

Returns:
    `{ok, app, key, url, notice?}` on create, or `{error, hint}`. The returned
    `url` is the link to the created item. A `notice` is the app stating what just
    became public and how to undo it, which is information the user needs to
    see rather than a detail to compress away. On a shape error the app names
    the offending field.