Add steering artifact receipt contract

[mdheller]

Summary

Adds the artifact receipt contract needed by Issue #34 before any local steering run can be accepted as auditable.

This PR does not close #34. It defines the receipt shape required before a future status: applied smoke record can be accepted.

What changed

• Adds contracts/steering-artifact-receipt.schema.json.
• Adds a pending fixture at examples/steering-artifact-receipts/gpt2-small-res-jb.pending.steering-artifact-receipt.json.
• Wires SteeringArtifactReceipt into native JSON validation.
• Adds docs/steering-artifact-receipts.md.
• Updates docs/steering-activation-path.md with the artifact receipt gate.
• Links the receipt doc from docs/index.md.

Receipt gate

A complete future receipt must record, for every model/tokenizer/SAE file used by the runtime:

• source repo
• exact file path
• resolved revision / commit / immutable tag
• local path
• file size
• SHA-256 digest
• digest verification status

Boundary discipline

This PR does not:

• download artifacts
• load GPT-2 Small
• load SAE artifacts
• emit a complete receipt
• return status: applied
• close Issue Implement controlled activation for local steering inference provider #34

Validation

Native CI passed on this PR head:

• validate run #166: passed
• Trust Surface run Plan Agent Machine runtime adapter for Superconscious ReasoningRun workloads #23: passed

The validate path checked the new SteeringArtifactReceipt schema and pending fixture.