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/app.css

@@ -1,68 +0,0 @@
-/* Import Claude theme from mini-lit */
-@import "@mariozechner/mini-lit/styles/themes/default.css";
-
-/* Tell Tailwind to scan mini-lit components */
-/* biome-ignore lint/suspicious/noUnknownAtRules: Tailwind 4 source directive */
-@source "../../../node_modules/@mariozechner/mini-lit/dist";
-
-/* Import Tailwind */
-/* biome-ignore lint/correctness/noInvalidPositionAtImportRule: fuck you */
-@import "tailwindcss";
-
-body {
-	font-size: 16px;
-	-webkit-font-smoothing: antialiased;
-}
-
-* {
-	scrollbar-width: thin;
-	scrollbar-color: var(--color-border) rgba(0, 0, 0, 0);
-}
-
-*::-webkit-scrollbar {
-	width: 8px;
-	height: 8px;
-}
-
-*::-webkit-scrollbar-track {
-	background: transparent;
-}
-
-*::-webkit-scrollbar-thumb {
-	background-color: var(--color-border);
-	border-radius: 4px;
-}
-
-*::-webkit-scrollbar-thumb:hover {
-	background-color: rgba(0, 0, 0, 0);
-}
-
-/* Fix cursor for dialog close buttons */
-.fixed.inset-0 button[aria-label*="Close"],
-.fixed.inset-0 button[type="button"] {
-	cursor: pointer;
-}
-
-/* Shimmer animation for thinking text */
-@keyframes shimmer {
-	0% {
-		background-position: -200% 0;
-	}
-	100% {
-		background-position: 200% 0;
-	}
-}
-
-.animate-shimmer {
-	animation: shimmer 2s ease-in-out infinite;
-}
-
-/* User message with fancy pill styling */
-.user-message-container {
-	transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
-	position: relative;
-	background: linear-gradient(135deg, rgba(217, 79, 0, 0.12), rgba(255, 107, 0, 0.12), rgba(212, 165, 0, 0.12));
-	border: 1px solid rgba(255, 107, 0, 0.25);
-	backdrop-filter: blur(10px);
-	max-width: 100%;
-}