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/widget-placement.ts

@@ -1,9 +0,0 @@
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function widgetPlacementExtension(pi: ExtensionAPI) {
-	pi.on("session_start", (_event, ctx) => {
-		if (!ctx.hasUI) return;
-		ctx.ui.setWidget("widget-above", ["Above editor widget"]);
-		ctx.ui.setWidget("widget-below", ["Below editor widget"], { placement: "belowEditor" });
-	});
-}