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.

docs/blacklist.md

@@ -5,15 +5,24 @@ exceptions, then ask whether a given text is blocked.
 
 | Command | Parameters | What it does |
 |---|---|---|
+| `/blacklist` | `[text...]` | Bare, lists both lists; with text, judges it |
 | `/blacklist_add` | `[text...]` | Adds text to the blacklist, or the message you replied to |
 | `/blacklist_del` | `<text...>` | Removes text from the blacklist |
 | `/whitelist_add` | `[text...]` | Adds an exception, or the message you replied to |
 | `/whitelist_del` | `<text...>` | Removes an exception |
 | `/blacklist_rules` | — | Lists both lists in one message |
 | `/blacklist_check` | `<text...>` | Judges a text against both lists |
+| `/whitelist_rnd` | — | Returns one whitelist entry at random |
 
 All are public and single-shot.
 
+`/blacklist` is the short form of the two read commands: on its own it does what
+`/blacklist_rules` does, and given text it does what `/blacklist_check` does. Nothing is
+lost by using it — the long names remain for when the intent should be explicit.
+
+`/whitelist_rnd` returns a single whitelist entry chosen at random, and says so plainly when
+the whitelist is empty rather than answering with nothing.
+
 ## The bot does not police the chat
 
 This is the first thing to know, because the module's name promises something it