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/rpc-jsonl.test.ts

@@ -1,65 +0,0 @@
-import { Readable } from "node:stream";
-import { describe, expect, test } from "vitest";
-import { attachJsonlLineReader, serializeJsonLine } from "../src/modes/rpc/jsonl.js";
-
-describe("RPC JSONL framing", () => {
-	test("serializes strict JSONL records without escaping Unicode separators", () => {
-		const line = serializeJsonLine({ text: "a\u2028b\u2029c" });
-
-		expect(line).toContain("a\u2028b\u2029c");
-		expect(line.endsWith("\n")).toBe(true);
-		expect(JSON.parse(line.trim())).toEqual({ text: "a\u2028b\u2029c" });
-	});
-
-	test("splits on LF only and preserves U+2028/U+2029 inside payloads", async () => {
-		const lines: string[] = [];
-		const stream = Readable.from([serializeJsonLine({ text: "a\u2028b\u2029c" })]);
-
-		const done = new Promise<void>((resolve) => {
-			stream.on("end", resolve);
-		});
-
-		attachJsonlLineReader(stream, (line) => {
-			lines.push(line);
-		});
-
-		await done;
-
-		expect(lines).toHaveLength(1);
-		expect(JSON.parse(lines[0])).toEqual({ text: "a\u2028b\u2029c" });
-	});
-
-	test("handles CRLF-delimited input", async () => {
-		const lines: string[] = [];
-		const stream = Readable.from([Buffer.from('{"a":1}\r\n{"b":2}\r\n')]);
-
-		const done = new Promise<void>((resolve) => {
-			stream.on("end", resolve);
-		});
-
-		attachJsonlLineReader(stream, (line) => {
-			lines.push(line);
-		});
-
-		await done;
-
-		expect(lines).toEqual(['{"a":1}', '{"b":2}']);
-	});
-
-	test("emits a final line without trailing LF", async () => {
-		const lines: string[] = [];
-		const stream = Readable.from([Buffer.from('{"a":1}')]);
-
-		const done = new Promise<void>((resolve) => {
-			stream.on("end", resolve);
-		});
-
-		attachJsonlLineReader(stream, (line) => {
-			lines.push(line);
-		});
-
-		await done;
-
-		expect(lines).toEqual(['{"a":1}']);
-	});
-});