feat: CLI with check/rules commands, formatters, integration tests

[hermanngeorge15]

Summary

• Text and JSON output formatters with unit tests
• Full CLI with check and rules commands (commander)
• Integration tests using execa (exit codes, JSON output, quiet mode)
• README with full usage section, rules table, and exit code docs

Exit codes

• 0: all checks passed
• 1: errors found
• 2: warnings only

Test plan

• 10 formatter unit tests (text: 6, json: 4)
• 5 CLI integration tests (valid file, invalid file, JSON output, quiet mode, rules list)
• Smoke tested check on both valid and invalid fixtures
• Smoke tested rules command
• All 70 tests pass

🤖 Generated with Claude Code