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/titlebar-spinner.ts

@@ -1,58 +0,0 @@
-/**
- * Titlebar Spinner Extension
- *
- * Shows a braille spinner animation in the terminal title while the agent is working.
- * Uses `ctx.ui.setTitle()` to update the terminal title via the extension API.
- *
- * Usage:
- *   pi --extension examples/extensions/titlebar-spinner.ts
- */
-
-import path from "node:path";
-import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
-
-const BRAILLE_FRAMES = ["⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏"];
-
-function getBaseTitle(pi: ExtensionAPI): string {
-	const cwd = path.basename(process.cwd());
-	const session = pi.getSessionName();
-	return session ? `π - ${session} - ${cwd}` : `π - ${cwd}`;
-}
-
-export default function (pi: ExtensionAPI) {
-	let timer: ReturnType<typeof setInterval> | null = null;
-	let frameIndex = 0;
-
-	function stopAnimation(ctx: ExtensionContext) {
-		if (timer) {
-			clearInterval(timer);
-			timer = null;
-		}
-		frameIndex = 0;
-		ctx.ui.setTitle(getBaseTitle(pi));
-	}
-
-	function startAnimation(ctx: ExtensionContext) {
-		stopAnimation(ctx);
-		timer = setInterval(() => {
-			const frame = BRAILLE_FRAMES[frameIndex % BRAILLE_FRAMES.length];
-			const cwd = path.basename(process.cwd());
-			const session = pi.getSessionName();
-			const title = session ? `${frame} π - ${session} - ${cwd}` : `${frame} π - ${cwd}`;
-			ctx.ui.setTitle(title);
-			frameIndex++;
-		}, 80);
-	}
-
-	pi.on("agent_start", async (_event, ctx) => {
-		startAnimation(ctx);
-	});
-
-	pi.on("agent_end", async (_event, ctx) => {
-		stopAnimation(ctx);
-	});
-
-	pi.on("session_shutdown", async (_event, ctx) => {
-		stopAnimation(ctx);
-	});
-}