feat: add spinloop work add #206

Append an item to the work items file the orchestrator works, from the shell — whether or not the orchestrator is running.

• Takes --items <path> (default ./work.yaml), the same file the orchestrator reads
• The item's fields — id, instructions, dir, tags, priority — come from flags; the file must stay parseable after the append, with the same validation the orchestrator applies
• An id the file already has is refused, naming it
• An id with a done or failed record in the state beside the file is refused too — reusing it would never work the item — naming the state
• A running orchestrator picks the new item up on its next pass; nothing more is needed

Part of the work command family with #207, #208, and #209. Builds on #205.