Document representative public validation pilot

[Brandon Miller (brandom-msft)]

Summary

• expose the manual representative public validation pilot from the root README
• document its four-sample coverage, invocation, reports, artifacts, and outcome contract
• record the maintenance steps and explicit non-gating, unscheduled, non-full-fleet limitations

Validation

• pilot producer and reporting contract suites (7 tests)
• tox -e typos for both changed Markdown files
• local documentation link checks (10 links)
• manifest, workflow matrix, sample path, and outcome vocabulary consistency checks
• live verification of successful run 31436177467 and its report/artifact names

Tracking

ADO 5510709 — https://msdata.visualstudio.com/Vienna/_workitems/edit/5510709