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/scripts/check-browser-smoke.mjs

@@ -1,36 +0,0 @@
-import { writeFileSync } from "node:fs";
-import { tmpdir } from "node:os";
-import { join } from "node:path";
-import { build } from "esbuild";
-
-const outputPath = join(tmpdir(), "pi-browser-smoke.js");
-const errorLogPath = join(tmpdir(), "pi-browser-smoke-errors.log");
-
-try {
-	await build({
-		entryPoints: ["scripts/browser-smoke-entry.ts"],
-		bundle: true,
-		platform: "browser",
-		format: "esm",
-		logLevel: "silent",
-		outfile: outputPath,
-	});
-	process.exit(0);
-} catch (error) {
-	let detailedErrors = "";
-	if (error && typeof error === "object" && "errors" in error && Array.isArray(error.errors)) {
-		detailedErrors = error.errors
-			.map((entry) => {
-				const location = entry.location
-					? `${entry.location.file}:${entry.location.line}:${entry.location.column}`
-					: "";
-				return [location, entry.text].filter(Boolean).join(" ");
-			})
-			.join("\n");
-	}
-
-	const baseError = error instanceof Error ? (error.stack ?? error.message) : String(error);
-	writeFileSync(errorLogPath, [detailedErrors, baseError].filter(Boolean).join("\n\n"), "utf-8");
-	console.error(`Browser smoke check failed. See ${errorLogPath}`);
-	process.exit(1);
-}