feat: add comprehensive integration test suite

29 integration tests covering all CLI subcommands (check, coverage,
generate, init), global flags (--strict, --require-coverage, --root),
multi-language support (TypeScript, Rust, Go, Python), and error cases
(missing files, invalid frontmatter, phantom exports, missing sections).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: corvid-agent