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/test/pi-user-agent.test.ts

@@ -0,0 +1,12 @@
+import { describe, expect, it } from "vitest";
+import { getPiUserAgent } from "../src/utils/pi-user-agent.ts";
+
+describe("getPiUserAgent", () => {
+	it("formats the user agent expected by pi.dev", () => {
+		const runtime = process.versions.bun ? `bun/${process.versions.bun}` : `node/${process.version}`;
+		const userAgent = getPiUserAgent("1.2.3");
+
+		expect(userAgent).toBe(`pi/1.2.3 (${process.platform}; ${runtime}; ${process.arch})`);
+		expect(userAgent).toMatch(/^pi\/[^\s()]+ \([^;()]+;\s*[^;()]+;\s*[^()]+\)$/);
+	});
+});