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/hello.ts

@@ -1,26 +0,0 @@
-/**
- * Hello Tool - Minimal custom tool example
- */
-
-import { Type } from "@mariozechner/pi-ai";
-import { defineTool, type ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-const helloTool = defineTool({
-	name: "hello",
-	label: "Hello",
-	description: "A simple greeting tool",
-	parameters: Type.Object({
-		name: Type.String({ description: "Name to greet" }),
-	}),
-
-	async execute(_toolCallId, params, _signal, _onUpdate, _ctx) {
-		return {
-			content: [{ type: "text", text: `Hello, ${params.name}!` }],
-			details: { greeted: params.name },
-		};
-	},
-});
-
-export default function (pi: ExtensionAPI) {
-	pi.registerTool(helloTool);
-}