[#2079] Use first-parent chain for next checkout

[alanpeixinho]

Parent

#2079
Schema: #2089
Backfill: #2091
Related: #1958

Issue extras “next checkout” currently picks the earliest later checkout on the same tree by start_time (#2054). That is CI clock order, not git history.

Once commits has parent data, use the first-parent chain instead.

In scope

• For last-seen checkout hash H, find a later checkout on the same tree/branch whose commit is the first-parent successor of H (commit whose first parent is H, or the next such commit that we actually ingested).
• Use parent storage from #2089 (ord = 0 or parent_hashes[0], depending on that decision).
• If the hash is missing from commits or there is no successor with a checkout, keep the current start_time behaviour.
• Same NextCheckout payload / API shape unless a field is clearly wrong.

Tests

Linear history: next checkout is the first-parent child, not an older start_time neighbour. Merge: first parent, not second. Missing commits row: fallback to start_time.