feat: sprint boards + Fibonacci story points

- Add board switcher UI plus "+ Board" / "+ Sprint" quick-create
  actions on the board page, backed by the existing multi-board
  support in pygitweb/tasks.py.
- Add /tasks/move endpoint and a "Move to board" control on task
  cards so tasks can be pulled from a backlog board into a sprint
  board (or back).
- Add a `points` field to Task (pygittools/tasks.py), constrained to
  the Fibonacci scale (0,1,2,3,5,8,13,21), editable from the card
  popout. Column headers show a running points total.
- Harden board name validation (boards_create, task_move) with
  pygit2.reference_is_valid_name instead of ad-hoc checks, since
  board names are literal git ref components; client-side board
  creation slugifies spaces to avoid tripping this.
- Tests for points create/update and task move between boards.