feat(heartbeat): add configurable visibility for heartbeat responses

Add per-channel and per-account heartbeat visibility settings:
- showOk: hide/show HEARTBEAT_OK messages (default: false)
- showAlerts: hide/show alert messages (default: true)
- useIndicator: emit typing indicator events (default: true)

Config precedence: per-account > per-channel > channel-defaults > global

This allows silencing routine heartbeat acks while still surfacing
alerts when something needs attention.

src/config/types.channels.ts

@@ -7,8 +7,19 @@ import type { TelegramConfig } from "./types.telegram.js";
 import type { WhatsAppConfig } from "./types.whatsapp.js";
 import type { GroupPolicy } from "./types.base.js";
 
+export type ChannelHeartbeatVisibilityConfig = {
+  /** Show HEARTBEAT_OK acknowledgments in chat (default: false). */
+  showOk?: boolean;
+  /** Show heartbeat alerts with actual content (default: true). */
+  showAlerts?: boolean;
+  /** Emit indicator events for UI status display (default: true). */
+  useIndicator?: boolean;
+};
+
 export type ChannelDefaultsConfig = {
   groupPolicy?: GroupPolicy;
+  /** Default heartbeat visibility for all channels. */
+  heartbeat?: ChannelHeartbeatVisibilityConfig;
 };
 
 export type ChannelsConfig = {