Q15: show which messages each agent actually read

Runs record a context manifest once a model turn succeeds; the chat
marks each human message with the agents that read its current version.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shared/domain.ts

@@ -151,6 +151,8 @@ export interface TaskRunRecord {
   startedAt: string | null;
   completedAt: string | null;
   errorMessage: string | null;
+  /** JSON RunContextManifest: which messages the run read. */
+  contextJson: string | null;
 }
 
 export interface ApprovalRecord {