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/doom-overlay/index.ts

@@ -1,74 +0,0 @@
-/**
- * DOOM Overlay Demo - Play DOOM as an overlay
- *
- * Usage: pi --extension ./examples/extensions/doom-overlay
- *
- * Commands:
- *   /doom-overlay - Play DOOM in an overlay (Q to pause/exit)
- *
- * This demonstrates that overlays can handle real-time game rendering at 35 FPS.
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-import { DoomOverlayComponent } from "./doom-component.js";
-import { DoomEngine } from "./doom-engine.js";
-import { ensureWadFile } from "./wad-finder.js";
-
-// Persistent engine instance - survives between invocations
-let activeEngine: DoomEngine | null = null;
-let activeWadPath: string | null = null;
-
-export default function (pi: ExtensionAPI) {
-	pi.registerCommand("doom-overlay", {
-		description: "Play DOOM as an overlay. Q to pause and exit.",
-
-		handler: async (args, ctx) => {
-			if (!ctx.hasUI) {
-				ctx.ui.notify("DOOM requires interactive mode", "error");
-				return;
-			}
-
-			// Auto-download WAD if not present
-			ctx.ui.notify("Loading DOOM...", "info");
-			const wad = args?.trim() ? args.trim() : await ensureWadFile();
-
-			if (!wad) {
-				ctx.ui.notify("Failed to download DOOM WAD file. Check your internet connection.", "error");
-				return;
-			}
-
-			try {
-				// Reuse existing engine if same WAD, otherwise create new
-				let isResume = false;
-				if (activeEngine && activeWadPath === wad) {
-					ctx.ui.notify("Resuming DOOM...", "info");
-					isResume = true;
-				} else {
-					ctx.ui.notify(`Loading DOOM from ${wad}...`, "info");
-					activeEngine = new DoomEngine(wad);
-					await activeEngine.init();
-					activeWadPath = wad;
-				}
-
-				await ctx.ui.custom(
-					(tui, _theme, _keybindings, done) => {
-						return new DoomOverlayComponent(tui, activeEngine!, () => done(undefined), isResume);
-					},
-					{
-						overlay: true,
-						overlayOptions: {
-							width: "75%",
-							maxHeight: "95%",
-							anchor: "center",
-							margin: { top: 1 },
-						},
-					},
-				);
-			} catch (error) {
-				ctx.ui.notify(`Failed to load DOOM: ${error}`, "error");
-				activeEngine = null;
-				activeWadPath = null;
-			}
-		},
-	});
-}