Use WaitGroup.Go for remaining manual Add/Done patterns

[pietern]

Summary

• Replace remaining wg.Add/wg.Done/go patterns with wg.Go(...) in libs/dagrun, libs/filer, and libs/sync
• These weren't caught by the use-waitgroup-go revive rule because they call Done() inside separate functions rather than inline

Test plan

• go test ./libs/dagrun/ ./libs/filer/ ./libs/sync/ passes

This pull request was AI-assisted by Isaac.

[github-actions]

Approval status: pending

/libs/filer/ - needs approval

Files: libs/filer/workspace_files_cache.go
Eligible: @simonfaltum, @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/libs/sync/ - needs approval

Files: libs/sync/sync.go
Eligible: @simonfaltum, @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

General files (require maintainer)

Files: libs/dagrun/dagrun.go
Based on git history:

• @denik -- recent work in libs/dagrun/

_{Any maintainer (@andrewnester, @anton-107, @denik, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
See OWNERS for ownership rules.}