Automates the Guild contributor program on project board #74 so the
cohort is recognized and kept unblocked without manual babysitting.

- Move a board issue to In Progress when a Guild member self-assigns it,
and post a friendly heads-up when they are assigned an issue that is not
on the board.
- Flag when a Guild member opens a new PR while another of theirs is
still open, to help them land work before spreading thin.
- Check in on quiet assignments and, if an assignee stays silent, free
the issue back to a to-do column so others can pick it up; a "guild
hold" label lets maintainers pause check-ins after they have followed
up.
- Share a weekly digest of what the Guild shipped.
- Label PRs from Guild members and recognize a Guild contributor tier on
the community PR board.

Release Notes:

- N/A