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/test/trigger-compact-extension.test.ts

@@ -1,57 +0,0 @@
-import { describe, expect, test, vi } from "vitest";
-import triggerCompactExtension from "../examples/extensions/trigger-compact.js";
-import type { ExtensionAPI, ExtensionCommandContext, ExtensionContext } from "../src/core/extensions/index.js";
-
-function createContext(tokens: number | null, compact = vi.fn()): ExtensionContext {
-	return {
-		hasUI: false,
-		ui: {} as ExtensionContext["ui"],
-		cwd: process.cwd(),
-		sessionManager: {} as ExtensionContext["sessionManager"],
-		modelRegistry: {} as ExtensionContext["modelRegistry"],
-		model: undefined,
-		isIdle: () => true,
-		signal: undefined,
-		abort: vi.fn(),
-		hasPendingMessages: () => false,
-		shutdown: vi.fn(),
-		getContextUsage: () => ({ tokens, contextWindow: 200_000, percent: tokens === null ? null : tokens / 2000 }),
-		compact,
-		getSystemPrompt: () => "",
-	};
-}
-
-describe("trigger-compact example extension", () => {
-	test("only auto-compacts when context usage crosses the threshold", () => {
-		let turnEndHandler:
-			| ((event: { type: "turn_end" }, ctx: ExtensionContext | ExtensionCommandContext) => void)
-			| undefined;
-
-		const api = {
-			on: (event: string, handler: (event: { type: "turn_end" }, ctx: ExtensionContext) => void) => {
-				if (event === "turn_end") {
-					turnEndHandler = handler;
-				}
-			},
-			registerCommand: vi.fn(),
-		} as unknown as ExtensionAPI;
-
-		triggerCompactExtension(api);
-		expect(turnEndHandler).toBeDefined();
-
-		const compact = vi.fn();
-		const event = { type: "turn_end" } as const;
-
-		turnEndHandler?.(event, createContext(110_000, compact));
-		expect(compact).not.toHaveBeenCalled();
-
-		turnEndHandler?.(event, createContext(120_000, compact));
-		expect(compact).not.toHaveBeenCalled();
-
-		turnEndHandler?.(event, createContext(95_000, compact));
-		expect(compact).not.toHaveBeenCalled();
-
-		turnEndHandler?.(event, createContext(105_000, compact));
-		expect(compact).toHaveBeenCalledTimes(1);
-	});
-});