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/tui/src/components/spacer.ts

@@ -1,28 +0,0 @@
-import type { Component } from "../tui.js";
-
-/**
- * Spacer component that renders empty lines
- */
-export class Spacer implements Component {
-	private lines: number;
-
-	constructor(lines: number = 1) {
-		this.lines = lines;
-	}
-
-	setLines(lines: number): void {
-		this.lines = lines;
-	}
-
-	invalidate(): void {
-		// No cached state to invalidate currently
-	}
-
-	render(_width: number): string[] {
-		const result: string[] = [];
-		for (let i = 0; i < this.lines; i++) {
-			result.push("");
-		}
-		return result;
-	}
-}