Set a memory node's visibility

Mark one entity node public or private.

Nodes are private until this is called — extraction never sets visibility, so nothing becomes public by inference. Marking a node public does not share it: it keeps its group and is still served to its owner alone. Cross-user sharing is what shared namespaces (graph_id) are for.

The agent_id path segment is accepted for URL symmetry with the other /memories/{agent_id}/... routes but does not narrow the scope — the node is matched across the caller's agents, because the memory browser that lists nodes is itself cross-agent. Another user's node is never reachable.

Responses: 404 when the node isn't in the requested scope; 422 MEMORY_NODE_PROTECTED for structural nodes (the user's own User node, Topic and Value taxonomy nodes) — the same set DELETE .../nodes/{id} refuses. Marking the User node public would make every fact hanging off it satisfy a visibility=public query at once.