Remove TODO.md and add testdata README (#121)

- Delete outdated TODO.md
- Add README to parser/testdata explaining that query.sql files
  are from ClickHouse repository under Apache 2.0 license

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

TODO.md

@@ -0,0 +1,11 @@
+# Test Data
+
+The `query.sql` files in this directory are sourced from the [ClickHouse](https://github.com/ClickHouse/ClickHouse) repository's test suite.
+
+## License
+
+The `query.sql` files are licensed under the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0), the same license as ClickHouse.
+
+The `explain.txt` and `explain_*.txt` files contain output generated by running ClickHouse's `EXPLAIN AST` command on the corresponding queries.
+
+All other files in this repository are subject to the repository's main license.