Plan: close workspaces from the GUI

Spec: spec.md Branch: codex/close-workspace

Design

Add a subtle close button to each persistent sidebar row and a Close Workspace entry to the central window-action registry. Both pass a captured workspace id to one App method. Show a single AdwAlertDialog with a destructive response; only its confirmation calls workspace.close. Refresh the authoritative workspace list after success, letting existing reconciliation select a remaining workspace or show the empty state.

Verification

Use the GUI's display-test seam with a fake IPC actor to check cancel, targeting, confirmation, and refresh. Run the workspace fmt, clippy, and test gates, then inspect the sidebar controls in light and dark mode if a display is available.

Constitution check

The spec precedes code; the GUI remains an IPC client; the existing server transition and event are reused. No new architectural decision is introduced.