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/ai/test/openai-codex-cache-affinity-e2e.test.ts

@@ -1,36 +0,0 @@
-import { describe, expect, it } from "vitest";
-import { getModel } from "../src/models.js";
-import { complete } from "../src/stream.js";
-import type { Context } from "../src/types.js";
-import { resolveApiKey } from "./oauth.js";
-
-const codexToken = await resolveApiKey("openai-codex");
-
-describe("openai-codex cache affinity e2e", () => {
-	it.skipIf(!codexToken)("handles SSE requests with aligned cache-affinity identifiers", async () => {
-		const model = getModel("openai-codex", "gpt-5.3-codex");
-		const sessionId = "0195d6e4-4cf9-7f44-a2d8-f8f7f49ee9d3";
-		const context: Context = {
-			systemPrompt: "You are a helpful assistant. Reply exactly as requested.",
-			messages: [
-				{
-					role: "user",
-					content: "Reply with exactly: cache affinity e2e success",
-					timestamp: Date.now(),
-				},
-			],
-		};
-
-		const response = await complete(model, context, {
-			apiKey: codexToken,
-			sessionId,
-			transport: "sse",
-		});
-
-		expect(response.stopReason, response.errorMessage).not.toBe("error");
-		expect(response.errorMessage).toBeUndefined();
-		expect(response.content.map((block) => (block.type === "text" ? block.text : "")).join("")).toContain(
-			"cache affinity e2e success",
-		);
-	});
-});