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/bun/cli.ts

@@ -1,6 +0,0 @@
-#!/usr/bin/env node
-process.title = "pi";
-process.emitWarning = (() => {}) as typeof process.emitWarning;
-
-await import("./register-bedrock.js");
-await import("../cli.js");

pi-mono-reference/packages/coding-agent/src/bun/register-bedrock.ts

@@ -1,4 +0,0 @@
-import { setBedrockProviderModule } from "@mariozechner/pi-ai";
-import { bedrockProviderModule } from "@mariozechner/pi-ai/bedrock-provider";
-
-setBedrockProviderModule(bedrockProviderModule);