refactor: use consistent path construction throughout codebase

[LudoLoops]

Issue

Path construction is inconsistent across the codebase:

Examples:

• task/create.go:28: Uses fmt.Sprintf("%s%s", path, name) ❌
• utils/storage.go:13: Uses filepath.Join(cfg.General.VaultPath, projectName) ✓
• task/finish.go:82: Uses utils.ListMarkdownFiles() ✓
• Other places mix approaches

Problem

• Some places use unsafe string concatenation
• Inconsistency makes code harder to follow
• Bug-prone (see Issue bug: ignored error in start command #4 about missing separators)

Solution

1. Always use filepath.Join for path construction
2. Create wrapper function in utils if needed:

func GetProjectDir(cfg config.Config, projectName string) string {
    return filepath.Join(cfg.General.VaultPath, projectName)
}

3. Audit entire codebase for consistency

Impact

🟢 Low - Code quality and safety