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/read-models/release.read-model.ts

@@ -8,6 +8,8 @@ export class ReleaseReadModel {
   targetDate!: string | null;
   status!: ReleaseStatus;
   releasedAt!: string | null;
+  /** Markdown, for people. */
+  notes!: string | null;
   createdAt!: string;
   version!: number;
 }