Add rm -r for folders, and make ls list one folder level

rm -r FOLDER deletes a folder and everything under it, records and
objects, so the space is freed. It lists what will go (the first 20
names, then a count) with the total size and asks on the terminal; no
answer or no terminal deletes nothing. -f skips the question and makes a
missing name a no-op, -rf and -fr combine them, and rm -r / is refused.
Objects still in use are worked out once per delete, not once per file.

ls now shows one level, like ls(1): sub-folders once with a trailing
slash, their total size and newest date, then the files, with names
relative to the folder. list keeps the recursive listing of full names
that scripts pipe through cut -c31-, and ls -r is the same.

Docs, man pages, the guide (new rm-r screenshot) and README follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

store.go

@@ -537,7 +537,53 @@ func (s *store) delete(name string) error {
 	if err != nil {
 		return err
 	}
-	return s.remove(cat, name, true)
+	return s.removeAll(cat, []string{name}, func(string) {})
+}
+
+// under returns the names in cat at or beneath the folder prefix, sorted.
+func under(cat map[string][]*record, prefix string) []string {
+	var names []string
+	for name := range cat {
+		if name == prefix || strings.HasPrefix(name, prefix+"/") {
+			names = append(names, name)
+		}
+	}
+	sort.Strings(names)
+	return names
+}
+
+// removeAll deletes every record for names and the objects they hold, then
+// drops them from cat, calling done after each name. Like remove, but it
+// works out once which objects other records still use, so deleting a large
+// folder stays linear.
+func (s *store) removeAll(cat map[string][]*record, names []string, done func(name string)) error {
+	gone := map[string]bool{}
+	for _, name := range names {
+		gone[name] = true
+	}
+	inUse := map[string]bool{}
+	for name, rs := range cat {
+		if !gone[name] {
+			for _, r := range rs {
+				inUse[r.Seed] = true
+			}
+		}
+	}
+	for _, name := range names {
+		for _, r := range cat[name] {
+			if err := s.removeFile(recordsDir, r.id); err != nil {
+				return err
+			}
+			if !inUse[r.Seed] {
+				if err := s.dropObjects(r.Seed, r.Size); err != nil {
+					return err
+				}
+			}
+		}
+		delete(cat, name)
+		done(name)
+	}
+	return nil
 }
 
 func (s *store) rename(oldName, newName string) error {