Migrating from GitHub carries its milestones into the forge's milestones, with their issues and pulls #3407

Replaces part 1 of #3288. The forge has a milestone noun now (src/core/milestones.ts, the milestones page), so GitHub milestones map onto it directly.

Done:

• [ ] A GitHub migration (the carry #3264 builds) brings each milestone: title, description, due date, open or closed.
• [ ] Each carried issue and pull that belonged to a milestone belongs to the same milestone here.
• [ ] Migrating twice does not duplicate milestones.
• [ ] Test: the fake GitHub (test/fake-origin.ts) seeded with 2 milestones (one closed) and 3 issues across them; one migration yields both milestones, their state and membership.

Files: the carry module #3264 adds, src/core/milestones.ts only if a create-with-due-date helper is missing, the migrate test. Waits for #3264: it adds the GitHub connection and the issue carry this extends.

Split into #3829 and #3830. A builder left this without a pull: carrying the milestones and putting issues and pulls into them was one change. Now #3829 carries the milestones themselves (title, description, due date, state, never twice), and #3830 adds each issue's and pull's membership. The blocker, #3264, is closed.