chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/coding-agent/src/extensions/index.ts

@@ -0,0 +1,4 @@
+import type { InlineExtension } from "../core/extensions/types.ts";
+import llamaExtension from "./llama/index.ts";
+
+export const builtInExtensions: InlineExtension[] = [{ name: "llama.cpp", factory: llamaExtension, hidden: true }];