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/examples/extensions/doom-overlay/wad-finder.ts

@@ -1,51 +0,0 @@
-import { existsSync, writeFileSync } from "node:fs";
-import { dirname, join, resolve } from "node:path";
-import { fileURLToPath } from "node:url";
-
-// Get the bundled WAD path (relative to this module)
-const __dirname = dirname(fileURLToPath(import.meta.url));
-const BUNDLED_WAD = join(__dirname, "doom1.wad");
-const WAD_URL = "https://distro.ibiblio.org/slitaz/sources/packages/d/doom1.wad";
-
-const DEFAULT_WAD_PATHS = ["./doom1.wad", "./DOOM1.WAD", "~/doom1.wad", "~/.doom/doom1.wad"];
-
-export function findWadFile(customPath?: string): string | null {
-	if (customPath) {
-		const resolved = resolve(customPath.replace(/^~/, process.env.HOME || ""));
-		if (existsSync(resolved)) return resolved;
-		return null;
-	}
-
-	// Check bundled WAD first
-	if (existsSync(BUNDLED_WAD)) {
-		return BUNDLED_WAD;
-	}
-
-	// Fall back to default paths
-	for (const p of DEFAULT_WAD_PATHS) {
-		const resolved = resolve(p.replace(/^~/, process.env.HOME || ""));
-		if (existsSync(resolved)) return resolved;
-	}
-
-	return null;
-}
-
-/** Download the shareware WAD if not present. Returns path or null on failure. */
-export async function ensureWadFile(): Promise<string | null> {
-	// Check if already exists
-	const existing = findWadFile();
-	if (existing) return existing;
-
-	// Download to bundled location
-	try {
-		const response = await fetch(WAD_URL);
-		if (!response.ok) {
-			throw new Error(`HTTP ${response.status}`);
-		}
-		const buffer = await response.arrayBuffer();
-		writeFileSync(BUNDLED_WAD, Buffer.from(buffer));
-		return BUNDLED_WAD;
-	} catch {
-		return null;
-	}
-}