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/example/index.html

@@ -1,13 +0,0 @@
-<!doctype html>
-<html lang="en">
-	<head>
-		<meta charset="UTF-8" />
-		<meta name="viewport" content="width=device-width, initial-scale=1.0" />
-		<title>Pi Web UI - Example</title>
-		<meta name="description" content="Example usage of @mariozechner/pi-web-ui - Reusable AI chat interface" />
-	</head>
-	<body class="bg-background">
-		<div id="app"></div>
-		<script type="module" src="/src/main.ts"></script>
-	</body>
-</html>