A box pass on an already merged pull reviews the merge's own diff, never “(no diff available)” #8739

Left out of #8682 (pull #8684, c172fe83): the once-only pass sends a landed-held issue to the crew reviewer on its merged pull, and it relies on the pull's objects still being in the repository. If they are gone, the reviewer sees "(no diff available)" and ticks blind.

Done looks like:

• [x] The review brief for a merged pull takes its diff from the merge commit on the base (first parent to merge), which is always present.
• [x] When no diff can be produced, the reviewer is not sent: the issue stays held, and the note says why.
• [x] A test merges a pull, deletes its head ref, runs the pass, and finds a real diff in the brief.

Files: src/crew/runner.ts, src/crew/brief.ts, src/actions/box_pass.ts, test/box-pass.test.ts Risks: a squash-merge has one parent. Diff it against its first parent.

Reviewed pull #8766 at its head: ticked The review brief for a merged pull takes its diff from the merge commit on the base (first parent to merge), which is always present.; When no diff can be produced, the reviewer is not sent: the issue stays held, and the note says why.; A test merges a pull, deletes its head ref, runs the pass, and finds a real diff in the brief..