Improve the app's workflow for forked repositories

Forked projects need support for choosing the upstream base, creating worktrees from the latest upstream commit, pushing branches to the fork, and opening pull requests against the upstream repository instead of assuming origin is always the source and target

Start by mapping the app's repository and pull-request workflow for forked repositories; the issue does not name files, tests, or entry points. Done means users can choose an upstream base, create worktrees from the latest upstream commit, push branches to the fork, and open pull requests against the upstream repository.

索引モデルが issue の本文から書いたものです。