[GA-M4][engineering] Generate GA evidence index from ga-eval artifacts

[Xunzhuo]

Updated from the GA issue triage. This issue is intended to be directly assignable.

Goal

Aggregate GA result artifacts into a single release evidence index.

Deliverables

• index.json with result ids, sources, verdicts, artifact dirs, and reproduction commands.
• Markdown summary grouped by milestone/source/priority.
• Links to Terminal-Bench, SR replay, SWE/internal, and final acceptance evidence.

Acceptance Criteria

• Index generation fails on missing or invalid result records.
• The markdown summary can be used by release notes.
• Every linked artifact path exists or is explicitly marked external/unavailable.