Changesets: staged deletes of gone records count as done, name stale ops

A web changeset with a staged record delete could not be applied after
the record was deleted elsewhere (e.g. with the CLI), and the error only
said "record N does not exist", so it was hard to find the operation to
remove. A delete of a record that is already gone is now a no-op with a
warning. Other operations that no longer apply fail with their number and
summary (op_summary in the API), and the Changes page marks that row and
offers to remove it from the error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/api.md

@@ -149,6 +149,14 @@ honours `If-Match: <version>`, and batch operations take `base_version`. If
 the object changed in the meantime the request fails with 409. Staged
 operations record the version they were staged against automatically.
 
+**Stale changesets.** When an object a staged operation needs was changed
+or deleted after it was staged (from the CLI, say), preview and apply fail
+on that operation: the error has `op` (its number in the changeset),
+`op_summary` and a message saying what happened; remove the operation
+(`DELETE /changesets/{id}/ops/{seq}`) or stage it again. A staged delete of
+a record that is already gone is not an error: it counts as done, with a
+warning.
+
 ## Resources
 
 `{zone}` is a zone ID or name (`root` for the root zone). Add `?view=NAME`