chore: drop non-code junk (pi-mono-reference, runtime state, root duplicates)

Remove files that should never have been tracked:

- pi-mono-reference/: 688-file copy of an upstream monorepo, zero imports,
  used only as offline human reference
- build/electron-builder.runtime.json: environment-specific build artifact
  with hardcoded local paths
- state/cancel-signal-state.json: runtime state written by the app
- prompt.txt, logo.svg (root): exact duplicates of the canonical copies
  under .planning/archive/ and assets/

Add state/ to .gitignore so future runtime state stays out.

pi-mono-reference/packages/ai/src/providers/github-copilot-headers.ts

@@ -1,37 +0,0 @@
-import type { Message } from "../types.js";
-
-// 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;
-}