feat(status): show the open PR count per member

jake status reported only branch and dirty-file count, so checking for
pending PRs meant visiting GitHub per repo. It now adds a PRS column,
filled from a single `gh search prs --owner sema-lisp` call rather than
one API call per member. Without gh, or unauthenticated, the column
shows "-" instead of 0.

The branch moved to the last column because branch names are the only
variable-width field; the fixed columns stay aligned.