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/src/utils/clipboard-native.ts

@@ -1,22 +0,0 @@
-import { createRequire } from "module";
-
-export type ClipboardModule = {
-	setText: (text: string) => Promise<void>;
-	hasImage: () => boolean;
-	getImageBinary: () => Promise<Array<number>>;
-};
-
-const require = createRequire(import.meta.url);
-let clipboard: ClipboardModule | null = null;
-
-const hasDisplay = process.platform !== "linux" || Boolean(process.env.DISPLAY || process.env.WAYLAND_DISPLAY);
-
-if (!process.env.TERMUX_VERSION && hasDisplay) {
-	try {
-		clipboard = require("@mariozechner/clipboard") as ClipboardModule;
-	} catch {
-		clipboard = null;
-	}
-}
-
-export { clipboard };