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/ai/src/api/github-copilot-headers.ts

@@ -0,0 +1,37 @@
+import type { Message } from "../types.ts";
+
+// Copilot expects X-Initiator to indicate whether the request is user-initiated
+// or agent-initiated (e.g. follow-up after assistant/tool messages).
+export function inferCopilotInitiator(messages: Message[]): "user" | "agent" {
+	const last = messages[messages.length - 1];
+	return last && last.role !== "user" ? "agent" : "user";
+}
+
+// Copilot requires Copilot-Vision-Request header when sending images
+export function hasCopilotVisionInput(messages: Message[]): boolean {
+	return messages.some((msg) => {
+		if (msg.role === "user" && Array.isArray(msg.content)) {
+			return msg.content.some((c) => c.type === "image");
+		}
+		if (msg.role === "toolResult" && Array.isArray(msg.content)) {
+			return msg.content.some((c) => c.type === "image");
+		}
+		return false;
+	});
+}
+
+export function buildCopilotDynamicHeaders(params: {
+	messages: Message[];
+	hasImages: boolean;
+}): Record<string, string> {
+	const headers: Record<string, string> = {
+		"X-Initiator": inferCopilotInitiator(params.messages),
+		"Openai-Intent": "conversation-edits",
+	};
+
+	if (params.hasImages) {
+		headers["Copilot-Vision-Request"] = "true";
+	}
+
+	return headers;
+}