Fix merge-aware Agentic TypeScript report validation

[noise64]

• validates newly introduced Agentic TypeScript reports against the exact report-bearing source tree for pull-request merges and ordinary two-parent main merges
• keeps direct and squash pushes strict against their resulting HEAD, while missing history, mismatched PR heads, failed Git selection, and ambiguous merge pushes fail closed
• adds a temporary-Git-repository contract test covering merge, direct-push, creation-event, and failure-path behavior
• preserves the historical September 7 reports instead of remeasuring them for an unrelated first-parent source change
• documents the merge-aware report-currentness contract