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/runtime/src/client/sessions/conversation.ts

@@ -96,6 +96,12 @@ export interface AssistantTiming {
 export interface AssistantMessageNode {
   kind: 'assistant'
   seq: number
+  /**
+   * Stable identity of the finalized model output, carried from the
+   * `assistant/message` event. Absent on interruption-frozen partials: those
+   * were never finalized, so they address no durable message.
+   */
+  messageId?: MessageId
   /** Unix epoch ms from the source session event (or turn/end when frozen from a partial). */
   time: number
   turn: number