feat(blacklist): add /blacklist shorthand and /whitelist_rnd

/blacklist is the short form of the two read commands: bare it lists both
lists like /blacklist_rules, and given text it judges that text like
/blacklist_check. Both long names stay for when the intent should be
explicit. An argument of only whitespace is not an argument, so it lists.

/whitelist_rnd returns one whitelist entry chosen at random, and says the
list is empty rather than answering with nothing. It reads only the
whitelist, and only for the calling topic.

The six existing command descriptions are shortened alongside. /help
renders as one un-chunked message pinned at 4096 runes, and two more
commands pushed it to 4123; the shorter wording brings it to 4049. That
ceiling is a shared limit this module did not create, and it will bind again
on the next command added anywhere in the repo.

cmd/server/command_menu_test.go

@@ -75,12 +75,14 @@ func TestCommandDiscovery_AllPublicCommandsHaveSafeMetadata(t *testing.T) {
 		"addsticker":           "[emoji...]",
 		"alias":                "<name>",
 		"aliases":              "",
+		"blacklist":            "[text...]",
 		"blacklist_add":        "[text...]",
 		"blacklist_del":        "<text...>",
 		"blacklist_rules":      "",
 		"blacklist_check":      "<text...>",
 		"whitelist_add":        "[text...]",
 		"whitelist_del":        "<text...>",
+		"whitelist_rnd":        "",
 		"unalias":              "<name>",
 		"insert":               "<name>",
 		"stats":                "[users | user <username> | cmd <command_name>]",

cmd/server/main_test.go

@@ -139,15 +139,15 @@ func TestFactoriesRegistersBlacklistCommands(t *testing.T) {
 		t.Fatalf("Build blacklist: %v", err)
 	}
 	for _, name := range []string{
-		"blacklist_add", "blacklist_del", "blacklist_rules", "blacklist_check",
-		"whitelist_add", "whitelist_del",
+		"blacklist", "blacklist_add", "blacklist_del", "blacklist_rules",
+		"blacklist_check", "whitelist_add", "whitelist_del", "whitelist_rnd",
 	} {
 		if _, ok := reg.AllCommands[name]; !ok {
 			t.Fatalf("missing command %s", name)
 		}
 	}
-	if got := len(reg.AllCommands); got != 6 {
-		t.Fatalf("blacklist registered %d commands, want 6", got)
+	if got := len(reg.AllCommands); got != 8 {
+		t.Fatalf("blacklist registered %d commands, want 8", got)
 	}
 }