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';