Implement a contribution in a fork

One contribution per run. The run's Multica issue names the target repository (owner/repo) and the change to make. There are two shapes. In the initial run, the deliverables are a pushed branch in the user's fork and a Multica comment carrying a pasteable PR body that follows the upstream PR template; no PR is opened anywhere — the user opens the upstream PR after validating the branch. In a review iteration (the PR is already open and maintainers left review comments), the deliverable is a push to the same fork branch plus a pasteable PR comment per PR on the Multica issue — and the PR discussion itself stays with the humans.

Scope

• In: verify the fork; read the upstream conventions; tests first; implement; run the upstream CI checks locally; commit with the correct sign-off; push the fork branch; write the PR body; in review iterations, address the explicit review comments on the same branch and write the pasteable PR comment; post the deliverable comment.
• Out (hard): creating the fork, opening a PR against the fork, opening a PR against upstream, merging anything, force-pushing, replying to or resolving PR review threads, editing the PR body, or deciding what an unresolved discussion means.