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

package.json

@@ -195,7 +195,6 @@
       "!stats/vite.config.ts",
       "!docs-site{,/**/*}",
       "!changes{,/**/*}",
-      "!backlog{,/**/*}",
       "!.tmp{,/**/*}",
       "!release-*{,/**/*}",
       "!dist/**/*.map",