fix(security): make allowFrom id-only by default with dangerous name opt-in (#24907)

* fix(channels): default allowFrom to id-only; add dangerous name opt-in

* docs(security): align channel allowFrom docs with id-only default

src/config/types.slack.ts

@@ -105,6 +105,11 @@ export type SlackAccountConfig = {
   userTokenReadOnly?: boolean;
   /** Allow bot-authored messages to trigger replies (default: false). */
   allowBots?: boolean;
+  /**
+   * Break-glass override: allow mutable identity matching (name/slug) in allowlists.
+   * Default behavior is ID-only matching.
+   */
+  dangerouslyAllowNameMatching?: boolean;
   /** Default mention requirement for channel messages (default: true). */
   requireMention?: boolean;
   /**