feat(engine): gate executor and heartbeat on allowParallelExecution

When a permanent agent has allowParallelExecution=false, TaskExecutor.execute()
defers if the agent has an active heartbeat run, and HeartbeatScheduler defers
a heartbeat if the agent's bound task has an active executor session. Each side
re-dispatches the other's deferred work on completion via resumeTaskForAgent
and the in-process runtime's onRunCompleted hook.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/runtimes/in-process-runtime.ts

@@ -448,6 +448,13 @@ export class InProcessRuntime
           onTerminated: (agentId, reason) => {
             runtimeLog.warn(`Agent ${agentId} terminated (unresponsive): ${reason}`);
           },
+          onRunCompleted: (agentId) => {
+            if (this.executor) {
+              void this.executor.resumeTaskForAgent(agentId).catch((err) => {
+                runtimeLog.warn(`resumeTaskForAgent failed for ${agentId}: ${err instanceof Error ? err.message : String(err)}`);
+              });
+            }
+          },
         });
         this.heartbeatMonitor.start();
       }