Add TestExplainVersionConsistency and integrate ClickHouse into regenerate-explain

- Add TestExplainVersionConsistency test that validates all explain*.txt files
  have consistent ClickHouse version headers
- Update cmd/regenerate-explain to download and manage ClickHouse v25.8.13.73-lts
  with HTTPS_PROXY support, idempotent start/stop, and version verification
- Update parser tests to strip version headers when comparing EXPLAIN output
- Remove scripts directory (functionality now in regenerate-explain command)

Author: claude