test: mutation testing — break dotup on purpose and check the suite notices

Each file in .tests/mutations/ is one deliberate bug: what it breaks, why that
matters, the assertion meant to catch it, and an OLD/NEW pair applied to a copy
of the tree. A mutation is KILLED only when the suite fails AND the named
assertion is among the failures — failing for an unrelated reason is reported
as WRONG-TEST, because that is luck, and luck is lost the next time the
unrelated test moves. A mutation whose OLD block no longer matches is STALE
rather than quietly skipped.

19 reproduces the DU-C1 shape: a comment moved back inside the fzf
line-continuation. It is killed by "dotup pick exits 0".

.tests/mutations/11-ensure-chezmoi-no-install.mut

@@ -0,0 +1,9 @@
+# what: ensure_chezmoi's installer call replaced with true
+# why:  the private tier died with `chezmoi: not found` AFTER writing the bws token
+# kills: the installer call did not install anything
+# run:  lab:scenarios/31-chezmoi-absent.sh
+<<<OLD
+	sh -c "$(curl -fsLS get.chezmoi.io)" -- -b "$HOME/.local/bin" >/dev/null 2>&1 \
+<<<NEW
+	true >/dev/null 2>&1 \
+<<<END