[Feature]: Workspace board — option to show agent sessions as individual cards (not only one card per worktree)

### Problem or use case

The workspace board (kanban) currently uses the worktree/workspace as the card unit. All agent sessions running in the same worktree are collapsed into a single card, so the board can only track progress per branch, not per task.

The intended workflow seems to be "one worktree per task", which makes the board meaningful — but this is impractical for large repositories, and that is the primary motivation for this request. Our project is a large Unity repository: every additional worktree costs tens of GB of disk and a full `Library` reimport that can take a long time. Teams like ours therefore work in a single long-lived worktree and run multiple agent sessions (one per task) inside it. On the board, all of that activity shows up as one card, so the kanban view provides almost no value.

Current behavior:
- Board cards = workspaces (worktrees). Sessions only appear as small agent rows inside a card.
- Board settings only manage status columns (add/remove/rename/move, Linear sync); there is no option to change the card unit.
- Status (`Todo` / `In progress` / ...) is stored as worktree metadata (`orca worktree set --workspace-status`), so sessions have no status of their own.

Environment: Orca 1.4.141, Windows 11, large Unity repository (single long-lived worktree workflow).

### Proposed solution

Add a board option such as "Card unit: workspace / agent session" (or "Group by: session"):

- Each agent session in a worktree becomes its own card, with its own board status.
- Cards could show the session title (first prompt / summary), agent type, and last activity — similar to the existing Agent Sessions history panel.
- Default can stay as-is (workspace cards); this would be opt-in for single-worktree workflows.

### Alternatives or additional context

_No response_