The first wt-clean.sh removed every worktree whose branch was merged into
origin/main — but long-lived branches (website, the ci line) are merged too,
so it would have deleted their worktrees. Caught on review before any real use.
Replace the auto-sweep with an explicit `wt-rm <branch>` that removes only the
named worktree; safety comes from git's own refusals — it won't remove a dirty
worktree (no --force) and deletes the local branch only when it's merged into
origin/main, otherwise keeps it (and its unmerged commits).
Update test-wt.sh to 15 checks (incl. "unnamed worktree untouched", primary
refused, dirty refused, unmerged branch preserved) and the wt-clean references
in ADR-0059 / CLAUDE.md / the README index.
