Walk Track: Add contract tests for NodePresenter JSON boundary

[nikhil2611]

Context

Walk Ex5 added 11-assertion contract tests for StatusPresenter#summarize_json with a golden file. NodePresenter (used by knife node show --format json) has no schema contract — shape regressions go undetected.

Code Paths

• lib/chef/knife/core/node_presenter.rb — format_for_display JSON path
• spec/unit/knife/core/node_presenter_spec.rb — add contract describe block
• spec/data/node_presenter_contract.json — new golden file

Acceptance Criteria

• At least 8 assertions covering required JSON keys (name, chef_environment, run_list, normal)
• Golden file committed with non-deterministic fields (ohai_time) zeroed or omitted
• Contract test runs in bundle exec rake spec with 0 failures
• ai-track-docs/contract-test.md updated with NodePresenter section

Dependencies

Follows pattern from Walk Ex5 (StatusPresenter contract). No blocking deps.

Size

Small — one PR.