Add missing workspace detection

[jongio]

Problem

Sessions whose working directories were deleted or moved still appear as normal. Launching them can fail in confusing ways because the terminal command starts from a path that no longer exists.

Proposed solution

Check whether each session cwd exists during refresh, show a Missing workspace badge in the list and preview, and add a status filter for these sessions. If a user tries to launch one, show a short error instead of starting a broken terminal command.

Acceptance criteria

• Sessions with missing cwd values show a distinct badge in the list and preview pane.
• The status picker includes a Missing workspace filter.
• Launching a missing workspace shows a clear error and does not start a terminal command.
• Refresh updates the badge when the directory returns.
• Tests cover existing paths, missing paths, and empty cwd values.

Notes

Complexity: M · Priority: 4