Before a fork's pull merges, its preview runs the combined build, and conflicts are resolved in a branch on our side with the decisions written for the author #9012

From #8935.

Done when:

• [x] A fork's pull with conflicts gets a branch on our side that merges main into the author's head; the crew resolves there, never on the author's branch.
• [x] The pull shows a note listing each conflicted file and what was kept, written for the author.
• [x] The preview is built from that combined branch, and the pull says so; the merge lands that branch so the author's head stays an ancestor.
• [x] A test: a conflicting fork pull yields the branch, the note and a preview of the combined tree.

Files: src/pulls/, the crew's resolve step, the preview source, a test.

Reviewed pull #9027 at its head: ticked A fork's pull with conflicts gets a branch on our side that merges main into the author's head; the crew resolves there, never on the author's branch.; The pull shows a note listing each conflicted file and what was kept, written for the author.; The preview is built from that combined branch, and the pull says so; the merge lands that branch so the author's head stays an ancestor.; A test: a conflicting fork pull yields the branch, the note and a preview of the combined tree..