Update from parent merges when the fork has diverged #4

A fork whose default branch has commits of its own cannot fast-forward to its parent, and today the page says 'merge by hand'. Make Update from parent do the merge instead: when the fork's tip is not an ancestor of the parent's, merge the parent's default branch into the fork's with a merge commit authored by the forge on behalf of the person who asked (the fast-forward stays the answer when it applies), refuse with a clear message when the merge would conflict, and leave the automatic following (core/following.ts) fast-forward-only as it is. Cover it in test/forks.test.ts: a diverged fork merges cleanly, a conflicting one is refused and left untouched. Keep the shape of src/actions/forks.ts and src/core/following.ts. Run the check command from forge.json before you push.

Ready again: pull #5 was closed because main moved under it and it could not be brought up to date. Conflicting files: src/actions/forks.ts, src/web/pages/repo.ts, test/forks.test.ts.

Parked by the crew: the check failed ("handover.drain lets go of the port before the rest of the drain, and closes only after the push is answered") and the decider saw no point in another try (pull #2122 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.