feat: expose context-engine compaction delegate helper (#49061)

* ContextEngine: add runtime compaction delegate helper

* plugin-sdk: expose compaction delegate through compat

* docs: clarify delegated plugin compaction

* docs: use scoped compaction delegate import

src/context-engine/index.ts

@@ -15,5 +15,6 @@ export {
 export type { ContextEngineFactory } from "./registry.js";
 
 export { LegacyContextEngine, registerLegacyContextEngine } from "./legacy.js";
+export { delegateCompactionToRuntime } from "./delegate.js";
 
 export { ensureContextEnginesInitialized } from "./init.js";