The guard at the worktree-merge resurrection block was inverting the
intended logic: it deleted any .planning/ file absent from PRE_MERGE_FILES,
which includes brand-new files (e.g. SUMMARY.md just created by the
executor). A genuine resurrection is a file that was previously tracked on
main, deliberately removed, and then re-introduced by the merge. Detecting
that requires a git history check — not just tree membership.
Fix: replace the PRE_MERGE_FILES grep guard with a `git log --follow
--diff-filter=D` check that only removes the file if it has a deletion
event in main's ancestry.
Closes#2501
