_fold_staged()/skill_pending_diff() folded patch ops with plain str.replace(),
ignoring replace_all and the fuzzy-match contract the approve path runs
(_patch_skill -> fuzzy_find_and_replace). A patch whose anchor repeats in the
target content previewed a folded diff ('X X') that /skills approve would then
reject with 'Found 2 matches'.
Fold previews through fuzzy_find_and_replace with the payload's replace_all and
render an explicit '(patch would fail: ...)' note instead of a fabricated
result; in a staged batch a failed patch leaves the later ops' base at the
pre-patch content, since approve aborts the batch at that op. Also drops the
trailing blank line at EOF flagged by git diff --check.
skill_pending_diff had no case for action='batch' (every skill_manage
operations[] write), so /skills diff <id> fell through to '(batch on …)'.
Batch ops apply in order: each op now diffs against what the earlier ops
of the same batch left in the file (disk only for untouched files), under
its skill_gist header; single-op paths are unchanged.
