fix: extension loading in Bun binary (#681)

- Remove loader exports from index.ts to break circular dependency
- Static import of index.js in loader.ts for virtualModules
- Add @mariozechner/pi-agent-core to virtualModules
- Update @mariozechner/jiti to 2.6.5 (bundles babel for Bun binary)
- Update SDK docs to use correct extension terminology

packages/coding-agent/src/index.ts

@@ -88,8 +88,6 @@ export type {
 	UserBashEventResult,
 } from "./core/extensions/index.js";
 export {
-	createExtensionRuntime,
-	discoverAndLoadExtensions,
 	ExtensionRunner,
 	isBashToolResult,
 	isEditToolResult,
@@ -98,7 +96,6 @@ export {
 	isLsToolResult,
 	isReadToolResult,
 	isWriteToolResult,
-	loadExtensions,
 	wrapRegisteredTool,
 	wrapRegisteredTools,
 	wrapToolsWithExtensions,