Several sessions at once is a surface property, not a session one

Answers the question 15 raised: a board showing many sessions leaves
one-per-node untouched, because each is still one conversation. Only
concurrent conversations with the same session would touch 0004.

Records soulstream and herdr as the prior art to draw from, and marks
it explicitly off the provisioning path so it stays a note rather than
becoming the work.

03-DESIGN/01-to-be/15-the-agent-session.md

@@ -174,6 +174,18 @@ That a person usually reaches the mesh session through a board is likely and is
 The session is reachable over the broker like everything else; what puts a text box in front of it
 is a different design, and the session does not know which surface asked.
 
+**Several sessions open at once is a property of the surface, not of the sessions.** A board
+showing the mesh's session beside one per node, switchable, leaves *one per node, permanent*
+untouched: each is still one conversation remembering all its callers together. Only *concurrent
+conversations with the same session* would touch ADR 0004, and that is not what is wanted.
+
+Such a surface is a **client** and holds nothing — it sends prompts and shows what the sessions
+themselves remember, which is what keeps it inside `11-a-board.md`'s rule that the board is not a
+second implementation of anything. Prior art to draw the interaction from: impire's *soulstream*
+(already cited in [ADR 0001](../../02-DECISIONS/0001-mesh-brokers-nodes-host-agents-think.md)) and
+`herdrdev/herdr`. **Not yet designed, and not on the path to replacing provisioning** — recorded
+here so the shape is not rediscovered.
+
 ## Consequences
 
 **Two sessions on one node, and no ambiguity.** The control-plane node hosts its own node session