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/sdk-session-manager.test.ts

@@ -1,66 +0,0 @@
-import { existsSync, mkdirSync, rmSync } from "node:fs";
-import { tmpdir } from "node:os";
-import { join } from "node:path";
-import { getModel } from "@mariozechner/pi-ai";
-import { afterEach, beforeEach, describe, expect, it } from "vitest";
-import { createAgentSession } from "../src/core/sdk.js";
-import { SessionManager } from "../src/core/session-manager.js";
-
-describe("createAgentSession session manager defaults", () => {
-	let tempDir: string;
-	let cwd: string;
-	let agentDir: string;
-
-	beforeEach(() => {
-		tempDir = join(tmpdir(), `pi-sdk-session-test-${Date.now()}-${Math.random().toString(36).slice(2)}`);
-		cwd = join(tempDir, "project");
-		agentDir = join(tempDir, "agent");
-		mkdirSync(cwd, { recursive: true });
-		mkdirSync(agentDir, { recursive: true });
-	});
-
-	afterEach(() => {
-		if (tempDir && existsSync(tempDir)) {
-			rmSync(tempDir, { recursive: true, force: true });
-		}
-	});
-
-	it("uses agentDir for the default persisted session path", async () => {
-		const model = getModel("anthropic", "claude-sonnet-4-5");
-		expect(model).toBeTruthy();
-
-		const { session } = await createAgentSession({
-			cwd,
-			agentDir,
-			model: model!,
-		});
-
-		const safePath = `--${cwd.replace(/^[/\\]/, "").replace(/[/\\:]/g, "-")}--`;
-		const expectedSessionDir = join(agentDir, "sessions", safePath);
-		const sessionDir = session.sessionManager.getSessionDir();
-		const sessionFile = session.sessionManager.getSessionFile();
-
-		expect(sessionDir).toBe(expectedSessionDir);
-		expect(sessionFile?.startsWith(`${expectedSessionDir}/`)).toBe(true);
-
-		session.dispose();
-	});
-
-	it("keeps an explicit sessionManager override", async () => {
-		const model = getModel("anthropic", "claude-sonnet-4-5");
-		expect(model).toBeTruthy();
-
-		const sessionManager = SessionManager.inMemory(cwd);
-		const { session } = await createAgentSession({
-			cwd,
-			agentDir,
-			model: model!,
-			sessionManager,
-		});
-
-		expect(session.sessionManager).toBe(sessionManager);
-		expect(session.sessionManager.isPersisted()).toBe(false);
-
-		session.dispose();
-	});
-});