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/web-ui/src/utils/format.ts

@@ -1,42 +0,0 @@
-import { i18n } from "@mariozechner/mini-lit";
-import type { Usage } from "@mariozechner/pi-ai";
-
-export function formatCost(cost: number): string {
-	return `$${cost.toFixed(4)}`;
-}
-
-export function formatModelCost(cost: any): string {
-	if (!cost) return i18n("Free");
-	const input = cost.input || 0;
-	const output = cost.output || 0;
-	if (input === 0 && output === 0) return i18n("Free");
-
-	// Format numbers with appropriate precision
-	const formatNum = (num: number): string => {
-		if (num >= 100) return num.toFixed(0);
-		if (num >= 10) return num.toFixed(1).replace(/\.0$/, "");
-		if (num >= 1) return num.toFixed(2).replace(/\.?0+$/, "");
-		return num.toFixed(3).replace(/\.?0+$/, "");
-	};
-
-	return `$${formatNum(input)}/$${formatNum(output)}`;
-}
-
-export function formatUsage(usage: Usage) {
-	if (!usage) return "";
-
-	const parts = [];
-	if (usage.input) parts.push(`↑${formatTokenCount(usage.input)}`);
-	if (usage.output) parts.push(`↓${formatTokenCount(usage.output)}`);
-	if (usage.cacheRead) parts.push(`R${formatTokenCount(usage.cacheRead)}`);
-	if (usage.cacheWrite) parts.push(`W${formatTokenCount(usage.cacheWrite)}`);
-	if (usage.cost?.total) parts.push(formatCost(usage.cost.total));
-
-	return parts.join(" ");
-}
-
-export function formatTokenCount(count: number): string {
-	if (count < 1000) return count.toString();
-	if (count < 10000) return `${(count / 1000).toFixed(1)}k`;
-	return `${Math.round(count / 1000)}k`;
-}