feat: add heartbeat visibility filtering for webchat

- Add isHeartbeat to AgentRunContext to track heartbeat runs
- Pass isHeartbeat flag through agent runner execution
- Suppress webchat broadcast (deltas + final) for heartbeat runs when showOk is false
- Webchat uses channels.defaults.heartbeat settings (no per-channel config)
- Default behavior: hide HEARTBEAT_OK from webchat (matches other channels)

This allows users to control whether heartbeat responses appear in
the webchat UI via channels.defaults.heartbeat.showOk (defaults to false).

src/infra/agent-events.ts

@@ -14,6 +14,7 @@ export type AgentEventPayload = {
 export type AgentRunContext = {
   sessionKey?: string;
   verboseLevel?: VerboseLevel;
+  isHeartbeat?: boolean;
 };
 
 // Keep per-run counters so streams stay strictly monotonic per runId.
@@ -34,6 +35,9 @@ export function registerAgentRunContext(runId: string, context: AgentRunContext)
   if (context.verboseLevel && existing.verboseLevel !== context.verboseLevel) {
     existing.verboseLevel = context.verboseLevel;
   }
+  if (context.isHeartbeat !== undefined && existing.isHeartbeat !== context.isHeartbeat) {
+    existing.isHeartbeat = context.isHeartbeat;
+  }
 }
 
 export function getAgentRunContext(runId: string) {