test(e2e): add comprehensive E2E test coverage for CLI

- Add Additional Slash Commands tests (/init, /bash, /feedback, /referral, /image, /exit)
- Add CLI Flags tests (--help, --version, --agent, invalid flags)
- Add Keyboard Interactions tests (Ctrl+C, @mentions, backspace, escape)
- Add Error Scenarios tests (low credits, invalid commands, edge cases)

Total: 30 E2E tests covering CLI functionality

Author: brandonkachen