Link protocol 3; a session the client opens runs on the chat's model or not at all

- LEMBAS_PROTOCOL, the device protocol and the protocols the CLI logs in to
  and links with are 3 alone (harness/acp.md *Versions*). A server listing
  only 1 and 2 is refused at login and at every dial — before anything is
  dialled — with its version and protocols and "update the server, or use
  LLeMbas CLI 1.x with it"; a newer one is told to update the CLI
  (ProtocolError). A 4400 stops the link with the same advice. The old
  downgrade to protocol 1 is gone.
- session/new's result says `instance`: whether the session's model is the
  link's instance's.
- `lembas_model` is the model, full stop: a device not logged in to the link's
  instance, a link with no instance (serve --stdio), or a model the instance
  does not serve to this device is -32602 with the reason, never a session on
  the device's own model. `_lembas/configure` naming such a model is refused
  likewise and the session keeps its model.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/lembas-login.test.ts

@@ -396,8 +396,12 @@ test("a denial, another service, and another protocol are each said plainly", as
   await expect(login(fake.url, { ...quick, onCode: () => {} })).rejects.toThrow("denied")
   fake.stop()
 
+  // A newer server: the CLI needs updating; a 1.x server (1 and 2): the server does (2.0 speaks 3 alone).
   fake = lembas({ protocol: PROTOCOL + 1 })
-  await expect(login(fake.url, { ...quick, onCode: () => {} })).rejects.toThrow("device protocol")
+  await expect(login(fake.url, { ...quick, onCode: () => {} })).rejects.toThrow("Update LLeMbas CLI")
+  fake.stop()
+  fake = lembas({ protocol: 2 })
+  await expect(login(fake.url, { ...quick, onCode: () => {} })).rejects.toThrow("needs a LLeMbas 2.0 server (protocol 3). Update the server, or use LLeMbas CLI 1.x with it.")
   fake.stop()
 
   const other = Bun.serve({ port: 0, fetch: () => new Response("hello", { status: 404 }) })