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/coding-agent/examples/extensions/notify.ts

@@ -1,55 +0,0 @@
-/**
- * Pi Notify Extension
- *
- * Sends a native terminal notification when Pi agent is done and waiting for input.
- * Supports multiple terminal protocols:
- * - OSC 777: Ghostty, iTerm2, WezTerm, rxvt-unicode
- * - OSC 99: Kitty
- * - Windows toast: Windows Terminal (WSL)
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-function windowsToastScript(title: string, body: string): string {
-	const type = "Windows.UI.Notifications";
-	const mgr = `[${type}.ToastNotificationManager, ${type}, ContentType = WindowsRuntime]`;
-	const template = `[${type}.ToastTemplateType]::ToastText01`;
-	const toast = `[${type}.ToastNotification]::new($xml)`;
-	return [
-		`${mgr} > $null`,
-		`$xml = [${type}.ToastNotificationManager]::GetTemplateContent(${template})`,
-		`$xml.GetElementsByTagName('text')[0].AppendChild($xml.CreateTextNode('${body}')) > $null`,
-		`[${type}.ToastNotificationManager]::CreateToastNotifier('${title}').Show(${toast})`,
-	].join("; ");
-}
-
-function notifyOSC777(title: string, body: string): void {
-	process.stdout.write(`\x1b]777;notify;${title};${body}\x07`);
-}
-
-function notifyOSC99(title: string, body: string): void {
-	// Kitty OSC 99: i=notification id, d=0 means not done yet, p=body for second part
-	process.stdout.write(`\x1b]99;i=1:d=0;${title}\x1b\\`);
-	process.stdout.write(`\x1b]99;i=1:p=body;${body}\x1b\\`);
-}
-
-function notifyWindows(title: string, body: string): void {
-	const { execFile } = require("child_process");
-	execFile("powershell.exe", ["-NoProfile", "-Command", windowsToastScript(title, body)]);
-}
-
-function notify(title: string, body: string): void {
-	if (process.env.WT_SESSION) {
-		notifyWindows(title, body);
-	} else if (process.env.KITTY_WINDOW_ID) {
-		notifyOSC99(title, body);
-	} else {
-		notifyOSC777(title, body);
-	}
-}
-
-export default function (pi: ExtensionAPI) {
-	pi.on("agent_end", async () => {
-		notify("Pi", "Ready for input");
-	});
-}