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/infra/heartbeat-visibility.ts

@@ -0,0 +1,58 @@
+import type { ClawdbotConfig } from "../config/config.js";
+import type { ChannelHeartbeatVisibilityConfig } from "../config/types.channels.js";
+import type { DeliverableMessageChannel } from "../utils/message-channel.js";
+
+export type ResolvedHeartbeatVisibility = {
+  showOk: boolean;
+  showAlerts: boolean;
+  useIndicator: boolean;
+};
+
+const DEFAULT_VISIBILITY: ResolvedHeartbeatVisibility = {
+  showOk: false, // Silent by default
+  showAlerts: true, // Show content messages
+  useIndicator: true, // Emit indicator events
+};
+
+export function resolveHeartbeatVisibility(params: {
+  cfg: ClawdbotConfig;
+  channel: DeliverableMessageChannel;
+  accountId?: string;
+}): ResolvedHeartbeatVisibility {
+  const { cfg, channel, accountId } = params;
+
+  // Layer 1: Global channel defaults
+  const channelDefaults = cfg.channels?.defaults?.heartbeat;
+
+  // Layer 2: Per-channel config (at channel root level)
+  const channelCfg = cfg.channels?.[channel] as
+    | {
+        heartbeat?: ChannelHeartbeatVisibilityConfig;
+        accounts?: Record<string, { heartbeat?: ChannelHeartbeatVisibilityConfig }>;
+      }
+    | undefined;
+  const perChannel = channelCfg?.heartbeat;
+
+  // Layer 3: Per-account config (most specific)
+  const accountCfg = accountId ? channelCfg?.accounts?.[accountId] : undefined;
+  const perAccount = accountCfg?.heartbeat;
+
+  // Precedence: per-account > per-channel > channel-defaults > global defaults
+  return {
+    showOk:
+      perAccount?.showOk ??
+      perChannel?.showOk ??
+      channelDefaults?.showOk ??
+      DEFAULT_VISIBILITY.showOk,
+    showAlerts:
+      perAccount?.showAlerts ??
+      perChannel?.showAlerts ??
+      channelDefaults?.showAlerts ??
+      DEFAULT_VISIBILITY.showAlerts,
+    useIndicator:
+      perAccount?.useIndicator ??
+      perChannel?.useIndicator ??
+      channelDefaults?.useIndicator ??
+      DEFAULT_VISIBILITY.useIndicator,
+  };
+}