Add comprehensive migration tests and coverage documentation

- Implemented `migrate-fixtures.test.ts` for validating migration fixtures with 29 tests covering spec-kit, OpenSpec, and ADR samples.
- Created `migrate-patterns.test.ts` to validate migration patterns with 24 tests focusing on structure, content, and migration steps.
- Enhanced test coverage with detailed documentation in `TEST-COVERAGE.md`, summarizing test files, coverage, and migration patterns.
- Ensured all tests pass with a total of 87 tests across multiple files, validating migration logic, content quality, and error handling.

Author: Marvin Zhang