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/auto-reply/reply/agent-runner-execution.ts

@@ -89,6 +89,7 @@ export async function runAgentTurnWithFallback(params: {
     registerAgentRunContext(runId, {
       sessionKey: params.sessionKey,
       verboseLevel: params.resolvedVerboseLevel,
+      isHeartbeat: params.isHeartbeat,
     });
   }
   let runResult: Awaited<ReturnType<typeof runEmbeddedPiAgent>>;