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/cancellable-loader.ts

@@ -1,40 +0,0 @@
-import { getKeybindings } from "../keybindings.js";
-import { Loader } from "./loader.js";
-
-/**
- * Loader that can be cancelled with Escape.
- * Extends Loader with an AbortSignal for cancelling async operations.
- *
- * @example
- * const loader = new CancellableLoader(tui, cyan, dim, "Working...");
- * loader.onAbort = () => done(null);
- * doWork(loader.signal).then(done);
- */
-export class CancellableLoader extends Loader {
-	private abortController = new AbortController();
-
-	/** Called when user presses Escape */
-	onAbort?: () => void;
-
-	/** AbortSignal that is aborted when user presses Escape */
-	get signal(): AbortSignal {
-		return this.abortController.signal;
-	}
-
-	/** Whether the loader was aborted */
-	get aborted(): boolean {
-		return this.abortController.signal.aborted;
-	}
-
-	handleInput(data: string): void {
-		const kb = getKeybindings();
-		if (kb.matches(data, "tui.select.cancel")) {
-			this.abortController.abort();
-			this.onAbort?.();
-		}
-	}
-
-	dispose(): void {
-		this.stop();
-	}
-}