doc: Add changelog entry

Author: effigies

changelog.d/20250822_091529_markiewicz_truncate_opaque_trees.md

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+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Added
+
+- A bullet item for the Added category.
+
+-->
+<!--
+### Changed
+
+- A bullet item for the Changed category.
+
+-->
+### Fixed
+
+- Avoid descending into opaque directories (such as `*.zarr/`) when checking
+  for unused sidecars. ([#227])
+
+[#227]: https://github.com/bids-standard/bids-validator/issues/227
+
+<!--
+### Deprecated
+
+- A bullet item for the Deprecated category.
+
+-->
+<!--
+### Removed
+
+- A bullet item for the Removed category.
+
+-->
+<!--
+### Security
+
+- A bullet item for the Security category.
+
+-->
+<!--
+### Infrastructure
+
+- A bullet item for the Infrastructure category.
+
+-->