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

scripts/verify-package-readme-model-experience.ts

@@ -70,6 +70,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/client/ui-layout': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/ui-sidebar': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/ui-conversation': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
+  'packages/client/ui-feedback': { kind: 'none', reason: 'Browser-side controls over the message-feedback sidecar; ratings and notes never enter the Session log, model context, or telemetry.' },
   'packages/client/ui-tool': { kind: 'none', reason: 'Browser-side Tool presentation layer; renders logged calls without changing model context.' },
   'packages/client/ui-deliverables': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/ui-task': { kind: 'none', reason: 'Browser-side read-only projection of ctx.tasks records; dsh-tool-tasks owns the model-facing behavior.' },