~anthropic/claude-sonnet-latest → B (6:4)

Side B fixes a genuine correctness/security bug: feed catch-up previously relied on wall-clock timestamps, which could miss or leak posts across users due to millisecond collisions or clock rollback, and adds strong multi-user private-room permission tests proving the anchor-by-ingest-position fix. Side A is a larger feature addition (GitHub import cards, resolver refactor) plus a real bug fix for the empty external garden index, but it's more feature/UI surface than a critical correctness fix, and part of the diff is churn from moving/renaming external_resolver.rs into resolvers/.