qa run --evidence record schema is undiscoverable: bare invalid_request and no schema docs #375

Split from #368 (item 3 of 3, per the fleet review recommending a per-subsystem split). Same secondary finding as #308.

Problem

The --evidence record schema accepted by cortex qa run is undiscoverable. A JSON file with a custom top-level shape ({"schema": ..., "candidate": ..., "evidence": [...]}) produces bare invalid_request, and cortex qa run --help documents neither the accepted record schema nor a pointer to where it lives.

Expected outcome

• --evidence validation failures report which field failed and why, not bare invalid_request.
• The evidence record schema is documented in qa run help or a referenced doc.

Affected scope

Evidence admission validation and its documentation only. Stage-attributed refusals for qa run are tracked in #374.

Refs #368, #308.