docs: add "Working with the agent" (norms for reading and instructing inferencebot)

A first draft of the member-facing norms listed as a design goal on
co-op/support#17: where the agent speaks and listens, what to ask it for,
what not to ask it in a shared room, what it does without confirmation,
what always needs a human, and how it is stopped. Linked from the README
index and the Matrix section of getting-started. Nothing here changes how
the agent is configured.

getting-started.md

@@ -135,6 +135,10 @@ privacy, and direction happen on our **Loomio forum**:
   space and join whatever is interesting to you. New rooms appear there as
   the co-op grows.
 
+- The updates feed is posted by the co-op's stewarding agent. See
+  [Working with the agent](working-with-the-agent.md) for what it does and
+  how members interact with it.
+
 - See the [Charter](charter.md) for how governance is structured.
 - See [Open questions](open-questions.md) for the decisions currently on the table.