fix(openapi-resource-gen): delete orphaned tag index.ts barrels on stale cleanup

Tag barrel files (e.g. actions/index.ts) were not included in the stale-file
tracking set, so removing all endpoints for a tag left behind an empty barrel
with broken re-exports. The fix:
- Expand preExistingFiles filter to include /index.ts and /index.mock.ts files
- Register every written barrel path in writtenFiles so root and active-tag
  barrels are protected; only barrels for tags no longer generated are deleted

Regenerates github-data-access to remove the 43 orphaned tag folders that
accumulated from a previous generation without --tagFilter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: constantant