feat: add vault path validation in config

[LudoLoops]

Issue

The VaultPath configuration is not validated when loaded or set.

Problem

• User might enter invalid paths (relative, non-existent, etc.)
• Silent failures occur if path doesn't exist
• No user feedback about bad configuration

Current Behavior

// In config.go - path accepted without validation
cfg.General.VaultPath = absPath  // No checks\!

Solution

Add validation function:

func validateVaultPath(path string) error {
    // Check if path is absolute
    if \!filepath.IsAbs(path) {
        return fmt.Errorf("vault path must be absolute, got: %s", path)
    }
    
    // Check if path exists
    if _, err := os.Stat(path); os.IsNotExist(err) {
        return fmt.Errorf("vault path does not exist: %s", path)
    }
    
    // Check if readable
    if err := os.Access(path, os.R_OK|os.W_OK); err \!= nil {
        return fmt.Errorf("vault path not readable/writable: %w", err)
    }
    
    return nil
}

Apply in LoadConfig() and on user input.

Benefits

• Catch configuration errors early
• Better user feedback
• Prevent silent failures

Impact

🟢 Low - UX improvement