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

package.json

@@ -40,6 +40,7 @@
 	},
 	"version": "0.0.3",
 	"dependencies": {
+		"@mariozechner/jiti": "^2.6.5",
 		"@mariozechner/pi-coding-agent": "^0.30.2",
 		"get-east-asian-width": "^1.4.0"
 	}