Define local steer endpoint contract stub

[mdheller]

Closes #32.

Summary

Defines a local Noetica-compatible /steer contract and adds a non-activating stub surface so Noetica can exercise the endpoint shape without real local SAE steering.

What changed

• Added docs/local-steer-endpoint.md.
• Linked the endpoint contract from docs/index.md.
• Added src/agent_machine/steering_stub.py.
• Added Python CLI commands:
  • agent-machine-py steer stub-response <request.json>
  • agent-machine-py steer serve-stub --host 127.0.0.1 --port 8080
• Added bootstrap CLI delegation:
  • agent-machine steer stub-response <request.json>
  • agent-machine steer serve-stub ...
• Updated make validate to exercise the non-server stub-response path.

Boundary discipline

This PR does not:

• load model weights
• fetch SAE artifacts
• register sourcesets
• implement activation injection
• start providers during validation
• claim production readiness

The stub returns Noetica-compatible SteeringResult JSON with status: not_configured or status: noop only.

Validation

Native CI passed on this PR head:

• validate run #160: passed
• Trust Surface run Add signed release bundle envelope and branch protection docs #17: passed

The validate path exercised the new steer stub-response command through both the Python CLI and bootstrap CLI paths.