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/tsconfig.base.json

@@ -1,23 +0,0 @@
-{
-	"compilerOptions": {
-		"target": "ES2022",
-		"module": "Node16",
-		"lib": ["ES2022"],
-		"strict": true,
-		"esModuleInterop": true,
-		"skipLibCheck": true,
-		"forceConsistentCasingInFileNames": true,
-		"declaration": true,
-		"declarationMap": true,
-		"sourceMap": true,
-		"inlineSources": true,
-		"inlineSourceMap": false,
-		"moduleResolution": "Node16",
-		"resolveJsonModule": true,
-		"allowImportingTsExtensions": false,
-		"experimentalDecorators": true,
-		"emitDecoratorMetadata": true,
-		"useDefineForClassFields": false,
-		"types": ["node"]
-	}
-}