feat(cli): add --check validation-only mode

[DiogoRibeiro7]

Summary

• add a new --check CLI flag to validate input without emitting converted output
• return non-zero when validation fails while preserving existing encode/decode error handling
• reject invalid flag combination: --check with --output
• add CLI integration tests for valid/invalid JSON and TOON in check mode
• document --check usage in README

Testing

• uv run pytest tests/test_cli.py -q (36 passed)