feat: check for required dependencies (gh, git) on startup

[LudoLoops]

Issue

If user doesn't have gh or proper git setup, errors are confusing and come too late.

Problem

• User runs tasky finish and gets: "gh pr merge: command not found"
• No clear indication that gh is required
• Setup errors appear at command time, not at startup

Solution

Add dependency check function:

func CheckDependencies() error {
    // Check git
    cmd := exec.Command("git", "--version")
    if err := cmd.Run(); err != nil {
        return fmt.Errorf("git not found: %w", err)
    }
    
    // Check gh (only if GitHub integration used)
    cmd = exec.Command("gh", "--version")
    if err := cmd.Run(); err != nil {
        return fmt.Errorf("gh CLI not found. Install it from: https://cli.github.com")
    }
    
    return nil
}

Call in main() or when first needed.

Benefits

• Clear error messages
• Early failure detection
• Better onboarding experience

Impact

🟢 Low - UX improvement