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/mac-system-theme.ts

@@ -1,47 +0,0 @@
-/**
- * Syncs pi theme with macOS system appearance (dark/light mode).
- *
- * Usage:
- *   pi -e examples/extensions/mac-system-theme.ts
- */
-
-import { exec } from "node:child_process";
-import { promisify } from "node:util";
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-const execAsync = promisify(exec);
-
-async function isDarkMode(): Promise<boolean> {
-	try {
-		const { stdout } = await execAsync(
-			"osascript -e 'tell application \"System Events\" to tell appearance preferences to return dark mode'",
-		);
-		return stdout.trim() === "true";
-	} catch {
-		return false;
-	}
-}
-
-export default function (pi: ExtensionAPI) {
-	let intervalId: ReturnType<typeof setInterval> | null = null;
-
-	pi.on("session_start", async (_event, ctx) => {
-		let currentTheme = (await isDarkMode()) ? "dark" : "light";
-		ctx.ui.setTheme(currentTheme);
-
-		intervalId = setInterval(async () => {
-			const newTheme = (await isDarkMode()) ? "dark" : "light";
-			if (newTheme !== currentTheme) {
-				currentTheme = newTheme;
-				ctx.ui.setTheme(currentTheme);
-			}
-		}, 2000);
-	});
-
-	pi.on("session_shutdown", () => {
-		if (intervalId) {
-			clearInterval(intervalId);
-			intervalId = null;
-		}
-	});
-}