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.ts

@@ -1,17 +0,0 @@
-#!/usr/bin/env node
-/**
- * CLI entry point for the refactored coding agent.
- * Uses main.ts with AgentSession and new mode modules.
- *
- * Test with: npx tsx src/cli-new.ts [args...]
- */
-process.title = "pi";
-process.env.PI_CODING_AGENT = "true";
-process.emitWarning = (() => {}) as typeof process.emitWarning;
-
-import { EnvHttpProxyAgent, setGlobalDispatcher } from "undici";
-import { main } from "./main.js";
-
-setGlobalDispatcher(new EnvHttpProxyAgent());
-
-main(process.argv.slice(2));