fix(checkpoints): surface skipped_oversize to users and stop misreporting failed deletes as restored

Follow-up to the salvaged #95207 fix, completing the misreport bug class:

- restore() now also drops delete_targets whose unlink failed (OSError
  swallowed) from restored_files — the sibling of the kept-oversize
  misreport the salvaged fix closed.
- /rollback output in the CLI (cli_commands_mixin) and gateway
  (slash_commands + gateway.rollback.kept_oversize locale key in all 17
  catalogs) now tells the user which files were kept because the size
  cap excluded them from every checkpoint; previously the file was
  correctly preserved but the user got no notice it was not reverted.
- Regression test for the failed-unlink misreport.

locales/zh-hant.yaml

@@ -280,6 +280,7 @@ Future messages in this room will use that transcript until `/reset` or another
     invalid_number:        "無效的檢查點編號。請使用 1-{max}。"
     restored:              "✅ 已還原至檢查點 {hash}：{reason}\n已自動儲存回復前的快照。"
     kept_user_edits:       "↷ 已保留您的手動編輯：{files}\n若也要還原這些檔案，請使用 /rollback <N> --all。"
+    kept_oversize:         "↷ 已保留（檔案過大無法納入檢查點，沒有可還原的儲存副本）：{files}"
     restore_failed:        "❌ {error}"
 
   diff: