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/utils/pi-user-agent.ts

@@ -0,0 +1,4 @@
+export function getPiUserAgent(version: string): string {
+	const runtime = process.versions.bun ? `bun/${process.versions.bun}` : `node/${process.version}`;
+	return `pi/${version} (${process.platform}; ${runtime}; ${process.arch})`;
+}