feat(config): gate channel config writes

src/config/types.signal.ts

@@ -8,6 +8,8 @@ export type SignalAccountConfig = {
   name?: string;
   /** Optional provider capability tags used for agent/runtime guidance. */
   capabilities?: string[];
+  /** Allow channel-initiated config writes (default: true). */
+  configWrites?: boolean;
   /** If false, do not start this Signal account. Default: true. */
   enabled?: boolean;
   /** Optional explicit E.164 account for signal-cli. */