When a fork's work lands, its author gets a thank-you note with what landed and what changed on the way #9013

From #8935.

Done when:

• [x] On landing a pull whose author lives on another forge (or a fork), a comment on the pull and, where the origin forge accepts it, on the origin pull thanks the author by name, links the landing commit and lists what changed from their head (conflicts resolved, slices).
• [x] Written once per landing, never for crew pulls.
• [x] A test covers the note's content and that it is sent once.

Files: src/origins/, the landing hook, a test.

Reviewed pull #9166 at its head: ticked On landing a pull whose author lives on another forge (or a fork), a comment on the pull and, where the origin forge accepts it, on the origin pull thanks the author by name, links the landing commit and lists what changed from their head (conflicts resolved, slices).; Written once per landing, never for crew pulls.; A test covers the note's content and that it is sent once..