Closes issue #117 (Metal Gear's feedback, 2026-10-18).
oxy merge oxygenhub/develop said 'no bookmark' though oxy log -r oxygenhub/develop worked: merge looked only at local bookmarks. It now also takes a remote's tracking bookmark (as last fetched), as revsets do, and says where it looked when neither exists.
- crates/oxy-core/src/repo/bookmarks.rs: merge resolves NAME locally, then with remote_bookmark().
- Test: a fresh clone merges origin/develop without switching; origin/nowhere is refused by name.
- Help and spec §10.1 say so. Gate green.