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/core/agent/src/types.ts

@@ -19,9 +19,10 @@
  *   live `Agent`. Two shapes: INTERCEPTION waterfalls (`agent/request`,
  *   `agent/step-result`, `agent/turn-continuation`) that mutate/veto, and
  *   TRANSIENT emits (`agent/status`, `agent/stream-chunk`, `agent/error`,
- *   `agent/created`/`agent/disposed`, `agent/queued`, `agent/steering`, and the
- *   turn boundaries) that notify with the `Agent` in hand. Answers "right now,
- *   with the agent object — intercept or observe."
+ *   `agent/created`/`agent/disposed`, `agent/queued`, `agent/steering`) that
+ *   notify with the `Agent` in hand. Turn/step boundaries are NOT here — they
+ *   are durable `session/event` records (see the rule below). Answers "right
+ *   now, with the agent object — intercept or observe."
  * - **`tools/*`** (`@deepseek-ai/dsh-tools`) — the tool registry + execution.
  *
  * **The rule:** a durable, replayable fact is a SessionEvent; a live