test: Expand test coverage

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Summary

Increase test coverage from 5 test files to 20+ test files.

Details

Following todoist-cli test patterns:

• Comprehensive API mocking with createMockApi() pattern
• Console output capturing for JSON validation
• Command parsing with program.exitOverride()

Priority Test Files to Add

1. document.test.ts - Full CRUD testing
2. collection.test.ts - Collection operations
3. refs.test.ts - Reference resolution (after feat: Add fuzzy reference resolution for documents and collections #10)
4. markdown.test.ts - Markdown rendering edge cases

Effort

3-5 days

Reference

Based on todoist-cli's 28 test files