feat: expose typed handoff reasoning effort

[Will-Liang329]

Adds optional reasoning_effort to the typed handoff_to_agent MCP input and serializes it as Reasoning effort: <value> in the existing deterministic handoff preamble.

The value is opaque metadata: CodexPro does not apply provider-specific effort validation or execute a provider. When omitted, the generated handoff retains its existing format and behavior. This supports downstream runner/router contracts without introducing an AHR dependency.

Validation: npm run build and the complete npm run smoke suite. The smoke test checks tools/list exposes the optional string field, verifies the serialized preamble, and verifies omission compatibility.

[rebel0789]

The implementation and complete local smoke suite pass, but one public contract is currently inaccurate: the reasoning_effort schema says the local executor validates provider support, while this PR only serializes opaque metadata and execute-handoff does not parse, apply, or validate it. Please either change that description to say it is metadata that CodexPro does not validate/apply, matching the PR body, or implement the claimed executor behavior. After #141 lands, please also update the branch so the patched dependency audit and Windows matrix can run cleanly.