Detect open parent drift during sync

[brdv]

Summary

Fixes #44 by making stck sync check every effective open parent→child edge with Git ancestry, even when PR base names already match.

When the first drifted child is found, sync rewrites it and cascades through its descendants while preserving local-only sync behavior and exact sync-time push leases.

Changelist

• Detect fetched parent/local child ancestry drift in both sync and status.
• Generalize sync planning to force rewrites from a non-first open branch.
• Add planner, status, reset, and real-Git sync/push regressions for advanced open parents.