Update CODEOWNERS for PR #990 (#991)

Assigns @labudis as codeowner for:
- /skills/issue-fields-migration/

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

CODEOWNERS

@@ -29,3 +29,6 @@
 /agents/doublecheck.agent.md @dvelton
 /plugins/doublecheck/ @dvelton
 /skills/doublecheck/ @dvelton
+
+# Added via #codeowner from PR #990
+/skills/issue-fields-migration/ @labudis