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/tsconfig.json

@@ -1,23 +0,0 @@
-{
-	"compilerOptions": {
-		"target": "ES2022",
-		"module": "ES2022",
-		"lib": ["ES2022", "DOM", "DOM.Iterable"],
-		"moduleResolution": "bundler",
-		"paths": {
-			"*": ["./*"],
-			"@mariozechner/pi-agent-core": ["../../agent/dist/index.d.ts"],
-			"@mariozechner/pi-ai": ["../../ai/dist/index.d.ts"],
-			"@mariozechner/pi-tui": ["../../tui/dist/index.d.ts"],
-			"@mariozechner/pi-web-ui": ["../dist/index.d.ts"]
-		},
-		"strict": true,
-		"skipLibCheck": true,
-		"esModuleInterop": true,
-		"allowSyntheticDefaultImports": true,
-		"experimentalDecorators": true,
-		"useDefineForClassFields": false
-	},
-	"include": ["src/**/*"],
-	"exclude": ["../src"]
-}