feat(feedback): add the Web surface for message feedback

Consume the durable message-feedback sidecar from #2217 in the browser:
per-message Like/Dislike with an optional note, contributed through a
declared assistant-actions slot.

- carry MessageId on finalized AssistantMessageNode so a target is nameable
- declare conversation.chat.assistant-actions and render it in the
  IconActions row between copy and branch
- hold one FeedbackController per Session with per-item ifVersion CAS,
  reconciling a version-conflict from the reply's authoritative item
- mount messageFeedbackRemote alongside goalsRemote

packages/client/ui-trajectory/src/client/trajectory-assistant-definition.ts

@@ -208,6 +208,7 @@ function finalNode(
     return {
       kind: 'assistant',
       seq: event.seq,
+      messageId: event.data.message.id,
       time: event.time,
       turn: state.turn,
       step: state.step,