Splits the 2,842-line apps/backend/internal/github/service.go monolith into nine focused files, all under the 800-line revive threshold. No public API changes, no behavior changes — Service struct, constructor, and every method signature preserved verbatim.
Files: errors.go (sentinels), service.go (struct + constructor + getters), service_tokens.go, service_pr.go, service_pr_watch.go, service_reviews.go, service_issues.go, service_cleanup.go, service_pr_status.go.
Only semantic-equivalent code rewrites: CheckPRWatch hasNew collapsed to a single || expression; three narrow //nolint directives for pre-existing complexity reflagged by golangci's new: true mode after the move.
Rebased onto current main after PR #1057 (sentinel extraction) landed — took its errors.go (now 4 sentinels incl. ErrInvalidToken) and ported the %w wraps for ConfigureToken and AssociateExistingPRByURL into the split files.
Henry