graph/ has walker primitives but no explicit merge-base API. Needed for PR "N ahead / M behind" UIs, three-way merge base selection, and rebase.

Scope

• Graph.merge_base(repo, a, b) :: {:ok, sha} | {:error, :disjoint}
• Graph.merge_bases/3 — multiple best-common-ancestors (for octopus / recursive merge)
• Graph.is_ancestor?(repo, a, b) — a reachable from b?
• Graph.ahead_behind(repo, a, b) :: %{ahead: n, behind: m}

Acceptance

• Matches git merge-base / git merge-base --all on a fixture with linear history, diamonds, and cross-merges.
• ahead_behind matches git rev-list --count --left-right.

Blocks: server-side merge primitives (separate ticket), PR UI.