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/cli/initial-message.ts

@@ -1,43 +0,0 @@
-import type { ImageContent } from "@mariozechner/pi-ai";
-import type { Args } from "./args.js";
-
-export interface InitialMessageInput {
-	parsed: Args;
-	fileText?: string;
-	fileImages?: ImageContent[];
-	stdinContent?: string;
-}
-
-export interface InitialMessageResult {
-	initialMessage?: string;
-	initialImages?: ImageContent[];
-}
-
-/**
- * Combine stdin content, @file text, and the first CLI message into a single
- * initial prompt for non-interactive mode.
- */
-export function buildInitialMessage({
-	parsed,
-	fileText,
-	fileImages,
-	stdinContent,
-}: InitialMessageInput): InitialMessageResult {
-	const parts: string[] = [];
-	if (stdinContent !== undefined) {
-		parts.push(stdinContent);
-	}
-	if (fileText) {
-		parts.push(fileText);
-	}
-
-	if (parsed.messages.length > 0) {
-		parts.push(parsed.messages[0]);
-		parsed.messages.shift();
-	}
-
-	return {
-		initialMessage: parts.length > 0 ? parts.join("") : undefined,
-		initialImages: fileImages && fileImages.length > 0 ? fileImages : undefined,
-	};
-}