Add demo that showcases RunProof verification workflow with broken app and run scripts

[sebamar88]

Motivation

• Provide a minimal, reproducible demo that shows RunProof blocking false completion claims by executing real verification commands.
• Include a tiny intentionally broken Node app and automated demo scripts for macOS/Linux and Windows to demonstrate the workflow end-to-end.

Description

• Add demo/README.md describing the demo and the intended flow, and add demo/.gitignore to ignore the disposable workspace /.demo-workdir/.
• Add the broken sample app under demo/broken-app/ with app.js (intentional bug), package.json, and test.js (assertion that initially fails).
• Add automated demo runners demo/scripts/run-demo.sh (executable) and demo/scripts/run-demo.ps1 that initialize a disposable RunProof workspace, reproduce the failing test, show RunProof blocking a fake completion, apply a one-line fix, and record passing evidence.

Testing

• No automated CI tests were added or run as part of this change.
• The demo includes a Node test (npm test --prefix demo/broken-app) which is intended to fail before the fix and pass after the scripted fix, and is exercised by the provided demo scripts when run locally.

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Codex Task