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.imessage.ts

@@ -4,6 +4,7 @@ import type {
   GroupPolicy,
   MarkdownConfig,
 } from "./types.base.js";
+import type { ChannelHeartbeatVisibilityConfig } from "./types.channels.js";
 import type { DmConfig } from "./types.messages.js";
 import type { GroupToolPolicyConfig } from "./types.tools.js";
 
@@ -63,6 +64,8 @@ export type IMessageAccountConfig = {
       tools?: GroupToolPolicyConfig;
     }
   >;
+  /** Heartbeat visibility settings for this channel. */
+  heartbeat?: ChannelHeartbeatVisibilityConfig;
 };
 
 export type IMessageConfig = {