fix(ui-stdio): seed turn labels from the registry; drop stale taxonomy line

Address review on the event-taxonomy PR:

- ui-stdio built its session-id→agent-id label map only from live
  `agent/created` events, so an agent registered before the UI fiber
  installed — the pre-created `main` agent, or any agent surviving an HMR
  reload of just this fiber — was missed and its turns rendered the raw
  session id instead of `[main turn N]`. Seed the map from
  `ctx.agents.list()` at install, then keep it live. Regression test proven
  red without the seed.
- The agent event-domain doc still listed "the turn boundaries" among the
  TRANSIENT `agent/*` emits, contradicting the rule ten lines below that a
  turn/step boundary is a durable `session/event`, not an `agent/*` mirror.

packages/support/ui-stdio/tests/readline.spec.ts

@@ -16,6 +16,9 @@ function fakeContext(): Context {
   return {
     on: vi.fn(() => vi.fn()),
     effect: vi.fn((callback: () => () => void) => callback()),
+    // The UI seeds its label map from the registry at install; this suite only
+    // exercises readline terminal-mode selection, so an empty roster suffices.
+    agents: { list: vi.fn(() => []) },
   } as unknown as Context
 }