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/tui/package.json

@@ -1,52 +0,0 @@
-{
-	"name": "@mariozechner/pi-tui",
-	"version": "0.67.5",
-	"description": "Terminal User Interface library with differential rendering for efficient text-based applications",
-	"type": "module",
-	"main": "dist/index.js",
-	"scripts": {
-		"clean": "shx rm -rf dist",
-		"build": "tsgo -p tsconfig.build.json",
-		"dev": "tsgo -p tsconfig.build.json --watch --preserveWatchOutput",
-		"test": "node --test --import tsx test/*.test.ts",
-		"prepublishOnly": "npm run clean && npm run build"
-	},
-	"files": [
-		"dist/**/*",
-		"README.md"
-	],
-	"keywords": [
-		"tui",
-		"terminal",
-		"ui",
-		"text-editor",
-		"differential-rendering",
-		"typescript",
-		"cli"
-	],
-	"author": "Mario Zechner",
-	"license": "MIT",
-	"repository": {
-		"type": "git",
-		"url": "git+https://github.com/badlogic/pi-mono.git",
-		"directory": "packages/tui"
-	},
-	"engines": {
-		"node": ">=20.0.0"
-	},
-	"types": "./dist/index.d.ts",
-	"dependencies": {
-		"@types/mime-types": "^2.1.4",
-		"chalk": "^5.5.0",
-		"get-east-asian-width": "^1.3.0",
-		"marked": "^15.0.12",
-		"mime-types": "^3.0.1"
-	},
-	"optionalDependencies": {
-		"koffi": "^2.9.0"
-	},
-	"devDependencies": {
-		"@xterm/headless": "^5.5.0",
-		"@xterm/xterm": "^5.5.0"
-	}
-}