Add dependency checks with helpful error messages

[robdimarco-atxp]

Summary

• Add dependency checking utility for git and npm before running CLI commands
• Show friendly error messages with installation instructions when dependencies are missing
• Add comprehensive tests for dependency checking functionality

Changes

• New utility: check-dependencies.ts - handles checking for external dependencies (git, npm)
• Enhanced CLI: Modified index.ts to check dependencies before running demo/create commands
• User experience: Clear error messages with installation links instead of cryptic spawn errors
• Tests: Added unit tests to verify dependency checking works correctly

Test plan

• Build and typecheck passes
• All existing tests pass
• New dependency checking tests pass
• Lint checks pass
• Manual testing confirms dependency checks work for demo command
• Help command works without dependency checks (as expected)

Fixes: https://linear.app/novellum/issue/ATXP-244/give-helpful-error-messages-if-pre-requisites-do-not-exist

🤖 Generated with Claude Code

[linear]

ATXP-244 Give helpful error messages if pre-requisites do not exist

We require tools such as git to be able to run the demo. We should check to see if they exist, and if not, prompt the user for it.