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/index.ts

@@ -24,11 +24,15 @@ export {
   type RuntimeMetrics,
   type ProjectRuntimeEvents,
   type GlobalMetrics,
+  type TaskExecutionResult,
+  type RuntimeHealth,
+  type RuntimeEventType,
 } from "./project-runtime.js";
 
 export { InProcessRuntime } from "./runtimes/in-process-runtime.js";
 export { ChildProcessRuntime } from "./runtimes/child-process-runtime.js";
 export { ProjectManager, type ProjectManagerEvents } from "./project-manager.js";
+export { HybridExecutor, type HybridExecutorEvents, type HybridExecutorOptions } from "./hybrid-executor.js";
 
 // ── IPC Protocol ───────────────────────────────────────────────────────