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/web-ui/example/package.json

@@ -1,25 +0,0 @@
-{
-  "name": "pi-web-ui-example",
-  "version": "1.55.5",
-  "private": true,
-  "type": "module",
-  "scripts": {
-    "dev": "vite",
-    "build": "vite build",
-    "preview": "vite preview",
-    "check": "tsgo --noEmit",
-    "clean": "shx rm -rf dist"
-  },
-  "dependencies": {
-    "@mariozechner/mini-lit": "^0.2.0",
-    "@mariozechner/pi-ai": "file:../../ai",
-    "@mariozechner/pi-web-ui": "file:../",
-    "@tailwindcss/vite": "^4.1.17",
-    "lit": "^3.3.1",
-    "lucide": "^0.544.0"
-  },
-  "devDependencies": {
-    "typescript": "^5.7.3",
-    "vite": "^7.1.6"
-  }
-}