Closes the agent+human JIT loop the control-pane board surfaces: the board shows
the unassigned (needs-owner) queue; 'claim' lets an agent or human pick up work.
node scripts/work-items.js claim [<id>] --owner <name> [--as agent|human]
- No id: claims the highest-priority unassigned open item.
- With id: claims that specific item (re-assignable).
- Sets owner, records metadata.assigneeKind (agent|human), and moves the card to
running so the board reflects that work has started.
- Refuses done items, requires --owner, validates --as. 5 CLI tests added.
Full suite 2844/2844; lint green.
