`agentws new --branch` always forked from the base branch, so a worktree for a
branch that already existed on origin started at the base's HEAD with none of
the branch's commits, and callers had to reset --hard afterwards to recover.
- Check out an existing origin/<branch> and set the branch to track it
- Fast-forward a stale local branch onto origin, keeping unpushed commits
- Fork from --from or the remote default only when origin lacks the branch
- Name which of the two paths was taken in the command's output
- Ignore --from, with a note, when the branch is already on origin
unkin-agent
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