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/bash-spawn-hook.ts

@@ -1,30 +0,0 @@
-/**
- * Bash Spawn Hook Example
- *
- * Adjusts command, cwd, and env before execution.
- *
- * Usage:
- *   pi -e ./bash-spawn-hook.ts
- */
-
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-import { createBashTool } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	const cwd = process.cwd();
-
-	const bashTool = createBashTool(cwd, {
-		spawnHook: ({ command, cwd, env }) => ({
-			command: `source ~/.profile\n${command}`,
-			cwd,
-			env: { ...env, PI_SPAWN_HOOK: "1" },
-		}),
-	});
-
-	pi.registerTool({
-		...bashTool,
-		execute: async (id, params, signal, onUpdate, _ctx) => {
-			return bashTool.execute(id, params, signal, onUpdate);
-		},
-	});
-}