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/provider-payload.ts

@@ -1,14 +0,0 @@
-import { appendFileSync } from "node:fs";
-import { join } from "node:path";
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: ExtensionAPI) {
-	const logFile = join(process.cwd(), ".pi", "provider-payload.log");
-
-	pi.on("before_provider_request", (event) => {
-		appendFileSync(logFile, `${JSON.stringify(event.payload, null, 2)}\n\n`, "utf8");
-
-		// Optional: replace the payload instead of only logging it.
-		// return { ...event.payload, temperature: 0 };
-	});
-}