Boards gain an optional project_id. When set, the board's default_workdir
mirrors the project's primary repo and every new task inherits the project —
a deterministic worktree + branch per task — unless it names its own. New
GET /projects; board create/patch/list carry project_id + resolved name; the
create dialog defaults its workspace to the board's and allows a per-task
path override. Desktop: "Board settings…" gains a project picker.
