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/agent/package.json

@@ -1,44 +0,0 @@
-{
-	"name": "@mariozechner/pi-agent-core",
-	"version": "0.67.5",
-	"description": "General-purpose agent with transport abstraction, state management, and attachment support",
-	"type": "module",
-	"main": "./dist/index.js",
-	"types": "./dist/index.d.ts",
-	"files": [
-		"dist",
-		"README.md"
-	],
-	"scripts": {
-		"clean": "shx rm -rf dist",
-		"build": "tsgo -p tsconfig.build.json",
-		"dev": "tsgo -p tsconfig.build.json --watch --preserveWatchOutput",
-		"test": "vitest --run",
-		"prepublishOnly": "npm run clean && npm run build"
-	},
-	"dependencies": {
-		"@mariozechner/pi-ai": "^0.67.5"
-	},
-	"keywords": [
-		"ai",
-		"agent",
-		"llm",
-		"transport",
-		"state-management"
-	],
-	"author": "Mario Zechner",
-	"license": "MIT",
-	"repository": {
-		"type": "git",
-		"url": "git+https://github.com/badlogic/pi-mono.git",
-		"directory": "packages/agent"
-	},
-	"engines": {
-		"node": ">=20.0.0"
-	},
-	"devDependencies": {
-		"@types/node": "^24.3.0",
-		"typescript": "^5.7.3",
-		"vitest": "^3.2.4"
-	}
-}