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/acp-v2.test.ts

@@ -82,12 +82,12 @@ async function client(limits?: Limits, o: { lembas?: boolean; protocol?: number;
 
 // §0
 
-test("initialize: protocol 2, the new capabilities, images taken, the login's connection named", async () => {
+test("initialize: protocol 3, the capabilities, images taken, the login's connection named", async () => {
   fake = fakeProvider([])
   config(fake.url)
   const root = project()
   const { init } = await client(limitsFor(root, { terminal: true }), { instance: "example" })
-  expect(init._meta.lembas.protocol).toBe(2)
+  expect(init._meta.lembas.protocol).toBe(3)
   for (const cap of ["files", "commands", "attachments", "ask", "plan", "model", "turns", "terminal", "shell"]) expect(init._meta.lembas.capabilities).toContain(cap)
   expect(init.agentCapabilities.promptCapabilities).toEqual({ image: true, audio: false, embeddedContext: true })
   expect(init._meta.lembas.connection).toBe("example")