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/coding-agent/tsconfig.examples.json

@@ -1,16 +0,0 @@
-{
-	"extends": "../../tsconfig.base.json",
-	"compilerOptions": {
-		"noEmit": true,
-		"paths": {
-			"@mariozechner/pi-coding-agent": ["./src/index.ts"],
-			"@mariozechner/pi-coding-agent/hooks": ["./src/core/hooks/index.ts"],
-			"@mariozechner/pi-tui": ["../tui/src/index.ts"],
-			"@mariozechner/pi-ai": ["../ai/src/index.ts"],
-			"@sinclair/typebox": ["../../node_modules/@sinclair/typebox"]
-		},
-		"skipLibCheck": true
-	},
-	"include": ["examples/**/*.ts"],
-	"exclude": ["node_modules", "dist"]
-}