Closes issue #100 (from oxygen_feedback.md; and it bit tonight's work too).
checkout --carry / bookmark switch --carry refused when uncommitted files were byte-for-byte what the target state holds, as after the proposal of that work landed, so the only way on was --discard, which agents rightly treat as destructive. A changed path whose file on disk is exactly the target's version (same bytes and exec bit), or a removed path the target lacks too, is now already applied: no conflict, nothing to carry; the other work still comes along, and a real difference is still refused.
Tests: carrying_work_the_target_already_has_is_no_conflict (new); merges_move_clean_working_copies_and_checkouts_say_what_blocks_them updated (its removed b.txt case is now this). Gate green (332 tests, clippy, fmt). Bookmark is main's heads plus this change.