CI: trigger workflow on PR description edits

Allow validate_datasets workflow to re-run when PR description is edited,
enabling dynamic detection of bids-specification-pr references added after
PR creation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: yarikoptic

.github/workflows/validate_datasets.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches: ['**']
   pull_request:
+    types: [opened, synchronize, reopened, edited]
     branches: ['**']
   create:
     branches: [master]