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/src/core/event-bus.ts

@@ -1,33 +0,0 @@
-import { EventEmitter } from "node:events";
-
-export interface EventBus {
-	emit(channel: string, data: unknown): void;
-	on(channel: string, handler: (data: unknown) => void): () => void;
-}
-
-export interface EventBusController extends EventBus {
-	clear(): void;
-}
-
-export function createEventBus(): EventBusController {
-	const emitter = new EventEmitter();
-	return {
-		emit: (channel, data) => {
-			emitter.emit(channel, data);
-		},
-		on: (channel, handler) => {
-			const safeHandler = async (data: unknown) => {
-				try {
-					await handler(data);
-				} catch (err) {
-					console.error(`Event handler error (${channel}):`, err);
-				}
-			};
-			emitter.on(channel, safeHandler);
-			return () => emitter.off(channel, safeHandler);
-		},
-		clear: () => {
-			emitter.removeAllListeners();
-		},
-	};
-}