chore: remove Backlog.md task system and backlog directory

- Delete Backlog.md and all backlog/ task files
- Remove Backlog.md MCP workflow instructions from AGENTS.md
- Drop "backlog history" from change-verification shim compatibility note

scripts/build-changelog.ts

@@ -613,8 +613,7 @@ function isIgnoredPullRequestPath(candidate: string): boolean {
     candidate === 'README.md' ||
     candidate.startsWith('changes/') ||
     candidate.startsWith('docs/') ||
-    candidate.startsWith('.github/') ||
-    candidate.startsWith('backlog/')
+    candidate.startsWith('.github/')
   );
 }