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/compaction-serialization.test.ts

@@ -1,79 +0,0 @@
-import type { Message } from "@mariozechner/pi-ai";
-import { describe, expect, it } from "vitest";
-import { serializeConversation } from "../src/core/compaction/utils.js";
-
-describe("serializeConversation", () => {
-	it("should truncate long tool results", () => {
-		const longContent = "x".repeat(5000);
-		const messages: Message[] = [
-			{
-				role: "toolResult",
-				toolCallId: "tc1",
-				toolName: "read",
-				content: [{ type: "text", text: longContent }],
-				isError: false,
-				timestamp: Date.now(),
-			},
-		];
-
-		const result = serializeConversation(messages);
-
-		expect(result).toContain("[Tool result]:");
-		expect(result).toContain("[... 3000 more characters truncated]");
-		expect(result).not.toContain("x".repeat(3000));
-		// First 2000 chars should be present
-		expect(result).toContain("x".repeat(2000));
-	});
-
-	it("should not truncate short tool results", () => {
-		const shortContent = "x".repeat(1500);
-		const messages: Message[] = [
-			{
-				role: "toolResult",
-				toolCallId: "tc1",
-				toolName: "read",
-				content: [{ type: "text", text: shortContent }],
-				isError: false,
-				timestamp: Date.now(),
-			},
-		];
-
-		const result = serializeConversation(messages);
-
-		expect(result).toBe(`[Tool result]: ${shortContent}`);
-		expect(result).not.toContain("truncated");
-	});
-
-	it("should not truncate assistant or user messages", () => {
-		const longText = "y".repeat(5000);
-		const messages: Message[] = [
-			{
-				role: "user",
-				content: [{ type: "text", text: longText }],
-				timestamp: Date.now(),
-			},
-			{
-				role: "assistant",
-				content: [{ type: "text", text: longText }],
-				api: "anthropic",
-				provider: "anthropic",
-				model: "test",
-				usage: {
-					input: 0,
-					output: 0,
-					cacheRead: 0,
-					cacheWrite: 0,
-					totalTokens: 0,
-					cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
-				},
-				stopReason: "stop",
-				timestamp: Date.now(),
-			},
-		];
-
-		const result = serializeConversation(messages);
-
-		expect(result).not.toContain("truncated");
-		expect(result).toContain(longText);
-	});
-});