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/session-info-modified-timestamp.test.ts

@@ -1,83 +0,0 @@
-import { writeFileSync } from "node:fs";
-import { stat } from "node:fs/promises";
-import { tmpdir } from "node:os";
-import { dirname, join } from "node:path";
-import { afterEach, beforeAll, describe, expect, it, vi } from "vitest";
-import type { SessionHeader } from "../src/core/session-manager.js";
-import { SessionManager } from "../src/core/session-manager.js";
-import { initTheme } from "../src/modes/interactive/theme/theme.js";
-
-function createSessionFile(path: string): void {
-	const header: SessionHeader = {
-		type: "session",
-		id: "test-session",
-		version: 3,
-		timestamp: new Date(0).toISOString(),
-		cwd: "/tmp",
-	};
-	writeFileSync(path, `${JSON.stringify(header)}\n`, "utf8");
-
-	// SessionManager only persists once it has seen at least one assistant message.
-	// Add a minimal assistant entry so subsequent appends are persisted.
-	const mgr = SessionManager.open(path);
-	mgr.appendMessage({
-		role: "assistant",
-		content: [{ type: "text", text: "hi" }],
-		api: "openai-completions",
-		provider: "openai",
-		model: "test",
-		usage: {
-			input: 1,
-			output: 1,
-			cacheRead: 0,
-			cacheWrite: 0,
-			totalTokens: 2,
-			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
-		},
-		stopReason: "stop",
-		timestamp: Date.now(),
-	});
-}
-
-describe("SessionInfo.modified", () => {
-	beforeAll(() => initTheme("dark"));
-
-	afterEach(() => {
-		vi.restoreAllMocks();
-	});
-
-	it("uses last user/assistant message timestamp instead of file mtime", async () => {
-		const filePath = join(tmpdir(), `pi-session-${Date.now()}-modified.jsonl`);
-		createSessionFile(filePath);
-
-		const before = await stat(filePath);
-		// Ensure the file mtime can differ from our message timestamp even on coarse filesystems.
-		await new Promise((r) => setTimeout(r, 10));
-
-		const mgr = SessionManager.open(filePath);
-		const msgTime = Date.now();
-		mgr.appendMessage({
-			role: "assistant",
-			content: [{ type: "text", text: "later" }],
-			api: "openai-completions",
-			provider: "openai",
-			model: "test",
-			usage: {
-				input: 1,
-				output: 1,
-				cacheRead: 0,
-				cacheWrite: 0,
-				totalTokens: 2,
-				cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
-			},
-			stopReason: "stop",
-			timestamp: msgTime,
-		});
-
-		const sessions = await SessionManager.list("/tmp", dirname(filePath));
-		const s = sessions.find((x) => x.path === filePath);
-		expect(s).toBeDefined();
-		expect(s!.modified.getTime()).toBe(msgTime);
-		expect(s!.modified.getTime()).not.toBe(before.mtime.getTime());
-	});
-});