caciarabot: the secret feature logs how many people, not which ones

The segreto feature in caciarabot keeps a roster of who has posted — display names only, no message content, because the Bot API cannot list a group’s membership — but its dry-run log line wrote those names out in full. It now logs a count instead (db905db), and the README’s Privacy section says so explicitly: the events say how many people a secret was about, never which ones. The change also cleared CodeQL alert py/clear-text-logging-sensitive-data, whose “sensitive data” label was firing on the word “secret” rather than on a credential — the dataflow was real even where the rule’s name was not, and the count is all the operator needs to see the feature working.