feat: release notes

Each release can carry notes in Markdown, written before or after it
ships. A draft is generated from its finished tickets grouped by type, to
edit and copy from the releases page.

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

libs/api/tracker/domain/src/lib/events/index.ts

@@ -37,3 +37,4 @@ export * from './sprint-started.event';
 export * from './sprint-closed.event';
 export * from './sprint-deleted.event';
 export * from './issue-sprint-changed.event';
+export * from './release-notes-written.event';

libs/api/tracker/domain/src/lib/events/release-notes-written.event.ts

@@ -0,0 +1,15 @@
+import { AutowiredEvent } from '@task-stream/api/shared/domain';
+import { TrackerEvent } from './tracker.event';
+
+/** The release notes were written or rewritten (null: removed). */
+@AutowiredEvent
+export class ReleaseNotesWrittenEvent extends TrackerEvent {
+  constructor(
+    actorId: string,
+    releaseId: string,
+    projectId: string,
+    readonly notes: string | null,
+  ) {
+    super(actorId, releaseId, projectId);
+  }
+}