feat: a saved A2UI card outlives its chat

[Lancetnik]

Is your feature request related to a problem?

A rendered card is not stored anywhere. durable_surfaces_from_messages() rebuilds every
A2UISurface from the chat's own message log on replay (gateway/core.py), and the data-model
updates a server action produces (A2UISurfaceDataUpdated) are events on that one chat's stream.

So a card exists only inside the thread that produced it. Ask the assistant in a new chat to
update the tracker it drew yesterday and there is nothing to update; have a scheduled task
refresh a dashboard card and it has no handle on it; delete the chat and the card is gone.

Describe the solution you'd like

Every card the agent creates becomes a file under profiles/<id>/files/ at the moment it is
created, and the thread renders that. One class of object — no "saved" versus "unsaved" card, no
pin gesture to remember.

Consequences that are the point:

• the card is visible in the Files tree like any other artifact, and reachable outside the chat
  it came from
• the agent can update its data model in a later session, or from a task run, by addressing the
  file
• a card can be read, diffed and copied between profiles

Storing them inside the agent's own file space (rather than a private sibling directory) is
deliberate: the agent reaches them with ordinary file tools, and so does the user.

Alternatives considered

• Only explicitly saved cards get a file — rejected: it splits cards into two classes and
  adds a gesture to remember.
• Snapshot-on-write archive, chat stays the source of truth — rejected: a dead copy solves
  reading, not updating, and updating is the ask.

Additional context

• src/assistant/gateway/core.py — _emit_a2ui_surfaces, durable_surfaces_from_messages
• src/assistant/a2ui.py — durable_surfaces_from_messages, update_data_value
• src/assistant/gateway/app.py — the server-action path that emits A2UISurfaceDataUpdated
• src/assistant/events.py — A2UISurface, A2UISurfaceDataUpdated
• ADR 0001 (single root directory), ADR 0012 (file references resolve by path)

Right now the way to reach a saved card outside a chat is the Files tree; a dedicated Live
Artifacts surface is tracked as a direction on the parent issue.

See also: #68 (tasks as editable files) — same instinct, a stored entity gets a file form.