fix: handle archived repositories during collaborator reconciliation - #38

Summary

Handle archived GitHub repositories gracefully during collaborator reconciliation.

Archived repositories are read-only in GitHub, so collaborator mutations return 403 and previously caused repeated reconciliation errors and retries.

Changes

• Detect archived repositories before attempting collaborator mutations.
• Skip collaborator create/update operations when the repository is archived.
• Set the resource condition to:
  • Ready=False
  • Reason=RepositoryArchived
• Requeue without returning an error, avoiding controller-runtime error backoff and repeated 403 logs.
• Handle the GitHub archived-repository 403 as a fallback in case the repository is archived between the initial read and the mutation.
• Preserve Revoke deletion semantics by keeping the finalizer until the repository becomes writable again.
• Add tests covering archived repository reconciliation and error classification.

Behavior

For an archived repository:

desired collaborator change
        ↓
repository archived
        ↓
mutation skipped
        ↓
Ready=False
Reason=RepositoryArchived
        ↓
scheduled requeue
        ↓
no reconciliation error