Add a LocalModelEngine conformance harness

[AIyahFun]

Create a runtime-neutral conformance harness for LocalModelEngine adapters.

The harness should measure skill selection, tool selection, argument validity, clarification behavior, cancellation, and unsafe-call rate without executing tools automatically.

Acceptance criteria:

• Uses only repository evaluation fixtures.
• Reports each metric separately.
• Rejects invalid structured output instead of repairing it into an action.
• Stores no prompt or response content in default logs.
• Includes a small fake-engine implementation for tests.