Add steering artifact resolver

[mdheller]

Summary

Adds an operator-controlled artifact resolver for the GPT-2 Small local steering sourceset.

What changed

• Adds src/agent_machine/steering_artifacts.py.
• Adds CLI command agent-machine steer resolve-artifacts.
• Adds CI-safe --dry-run mode that emits a pending receipt without network access.
• Adds operator mode with --allow-network for Hugging Face resolution and download into an explicit --local-dir.
• Emits SteeringArtifactReceipt JSON to --receipt-out.
• Computes SHA-256 digest for each resolved file.
• Captures source repo, exact file path, resolved immutable revision, local path, size, and digest verification status.
• Updates make validate to exercise dry-run receipt generation.
• Adds docs/steering-artifact-resolution.md and links it from the docs index.

Boundary discipline

This PR does not load a model, load an SAE, run inference, inject activations, produce an applied steering result, or claim runtime readiness.

Validation

Native CI passed on this PR head:

• validate run #168: passed
• Trust Surface run Implement RuntimeInstallReceipt emission for runtime install/update flows #25: passed

The validate path exercised steer resolve-artifacts --dry-run and validated the emitted receipt shape.