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/scripts/generate-test-image.ts

@@ -1,34 +0,0 @@
-#!/usr/bin/env tsx
-
-import { createCanvas } from "canvas";
-import { writeFileSync } from "fs";
-import { join, dirname } from "path";
-import { fileURLToPath } from "url";
-
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = dirname(__filename);
-
-// Create a 200x200 canvas
-const canvas = createCanvas(200, 200);
-const ctx = canvas.getContext("2d");
-
-// Fill background with white
-ctx.fillStyle = "white";
-ctx.fillRect(0, 0, 200, 200);
-
-// Draw a red circle in the center
-ctx.fillStyle = "red";
-ctx.beginPath();
-ctx.arc(100, 100, 50, 0, Math.PI * 2);
-ctx.fill();
-
-// Save the image
-const buffer = canvas.toBuffer("image/png");
-const outputPath = join(__dirname, "..", "test", "data", "red-circle.png");
-
-// Ensure the directory exists
-import { mkdirSync } from "fs";
-mkdirSync(join(__dirname, "..", "test", "data"), { recursive: true });
-
-writeFileSync(outputPath, buffer);
-console.log(`Generated test image at: ${outputPath}`);