feat(KB-501): implement multi-project runtime architecture with HybridExecutor

- Add ProjectRuntime interface with InProcessRuntime and ChildProcessRuntime implementations
- Implement HybridExecutor for multi-project task orchestration
- Add IPC protocol for host-worker communication in child-process mode
- Implement health monitoring with automatic restart for child processes
- Add comprehensive tests for hybrid-executor and project-runtime
- Update AGENTS.md with multi-project runtime documentation
- Add changeset for hybrid-executor-runtime release

packages/engine/src/project-runtime.ts

@@ -128,6 +128,53 @@ export interface ProjectRuntime extends EventEmitter<ProjectRuntimeEvents> {
   getMetrics(): RuntimeMetrics;
 }
 
+/**
+ * Result of a task execution operation.
+ */
+export interface TaskExecutionResult {
+  /** Whether the task execution was successful */
+  success: boolean;
+  /** The task ID that was executed */
+  taskId: string;
+  /** Error message if execution failed */
+  error?: string;
+  /** Number of steps completed during execution */
+  stepsCompleted?: number;
+  /** ISO-8601 timestamp when execution started */
+  startedAt?: string;
+  /** ISO-8601 timestamp when execution completed */
+  completedAt?: string;
+}
+
+/**
+ * Health metrics for a ProjectRuntime instance.
+ * Used for detailed health monitoring and diagnostics.
+ */
+export interface RuntimeHealth {
+  /** Current status of the runtime */
+  status: RuntimeStatus;
+  /** Number of tasks currently in-progress */
+  activeTasks: number;
+  /** Memory usage in bytes */
+  memoryUsage: number;
+  /** ISO-8601 timestamp of the last activity */
+  lastActivityAt: string;
+  /** Number of errors encountered */
+  errorCount: number;
+  /** Optional last error message */
+  lastError?: string;
+}
+
+/**
+ * Runtime event types for event handlers.
+ */
+export type RuntimeEventType =
+  | "task:created"
+  | "task:completed"
+  | "task:failed"
+  | "health:changed"
+  | "error";
+
 /**
  * Global metrics aggregated across all project runtimes.
  */