Electron App: Demo mode

[jongio]

Overview

Synthetic data mode for experimentation and screenshots, matching TUI's --demo flag.

Acceptance Criteria

• Launch with --demo flag or menu option generates synthetic sessions
• Synthetic data covers: multiple repos, branches, statuses, turn counts
• Demo data includes conversations, checkpoints, files, refs
• Demo mode indicator visible in title bar
• No writes to real session store in demo mode
• Useful for screenshots, demos, and new user onboarding
• Same synthetic data as TUI demo (shared test fixtures)