feat: add --on-hold flag to card move command

[nnemirovsky]

Adds support for moving cards to the on-hold section of a column via bc4 card move <id> --on-hold.

The Basecamp API represents on-hold as a sub-container (Kanban::OnHold) within a column, with its own ID. The standard card move endpoint accepts this ID as column_id, so no new API calls are needed — just the on-hold section ID from the card table response.

When --on-hold is used alone, the card moves to on-hold in its current column. Combined with --column, it moves to on-hold in the specified column.

Also adds ID field to OnHoldStatus struct — the API returns it but it wasn't being captured.

Depends on #142 (on-hold cards URL fix).

bc4 card move 123 --on-hold
bc4 card move 123 --column "Developing" --on-hold

[brigleb]

Clean feature addition. Reuses the existing MoveCard API with the on-hold container ID — nice approach. Branch rebased onto main to drop the already-merged #142 commit. LGTM.