fix: report output format failures as usage errors

[codeforester]

Summary

• Classify public OutputFormatError failures at the run_app process boundary as exit-code 2 usage errors.
• Preserve safe actionable dependency/format messages in human output and use output_format_error in JSON mode.
• Add process-boundary tests for optional YAML dependency failures and document the contract.

Closes #343

Validation

• uv run --extra dev --extra typer --extra quality python -m pytest tests/test_optional_yaml_dependency.py tests/test_json_contracts.py -q
• Ruff check and format check for changed Python files
• git diff --check