feat(cli): add --json to steer-report for machine-readable output paths

[Nitjsefnie]

Fixes #9.

steer-report --json prints the outputs mapping as a single JSON object (path values as strings) to stdout and suppresses the human [steerbench] wrote: block; the default path is byte-for-byte unchanged:

{"markdown": ".../report.md", "html": ".../report.html", "dose_png": ".../report_dose.png", "layer_png": ".../report_layer.png"}

Test: test_cli_json_emits_artifact_paths, a sibling of test_cli_renders_from_real_artifacts with the same importorskip("matplotlib") guard — parses stdout with json.loads, asserts all four artifact keys point at existing files, and asserts the human block is absent. Mutation-verified (re-run independently before opening): letting --json also print the human lines fails the test; dropping a key from the emitted dict fails the four-keys assertion.

All four gates green, baseline vs branch: ruff format --check, ruff check src tests, mypy src, pytest (58 passed / 3 skipped without matplotlib — the +1 skip is the new guarded test; 60 passed / 1 skipped with matplotlib via uv run --with matplotlib, which leaves uv.lock untouched). Commit contains exactly the two intended files.

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Generated by Claude Fable 5 (brief, review), Claude Opus 4.8 (1M context) (implementation)

[bamdadd]

Verified locally — ruff/format/mypy all clean and the new test passes (with matplotlib: json parses, all four artifact keys point at real files, and the human [steerbench] wrote: block is correctly suppressed). The default text path is byte-for-byte unchanged since the else branch is identical to before. Clean, well-scoped addition — merging. Thanks Peter.