Propose upstream across forges, part 2: from a child's landed change, one button pushes it to the parent and opens the pull, and the child shows its state #1832

Split from #424. Done looks like:

• [ ] On a child forge, extending src/actions/upstream.ts: when the parent is another forge, one button pushes the change to child/<name>/<branch> at the parent with the proposal token and opens the pull there, naming the source change.
• [ ] The child shows the proposal as open, merged or closed, read from the parent and cached; an unreachable parent shows "parent unreachable", not an error page.
• [ ] Tests with a fake parent: a proposal is pushed and opened, its state is read back, and an unreachable parent degrades to one line.