feat(ui-workspace): archive session from the row menu

The visual-only Delete session placeholder becomes a wired Archive
session action: no confirmation dialog (non-destructive), failures stay
console diagnostics. tree.ts hides archived sessions in every
derivation (workspace groups, Ungrouped, search, flat list) through the
sessionVisible predicate. The workspace-management e2e pins the archive
round trip across reload.

packages/client/ui-workspace/tests/workspace-picker.spec.tsx

@@ -32,7 +32,7 @@ const sessions: SessionListState = {
   ids: [], byId: {}, current: undefined, phase: 'ready',
 }
 const workspaceState = (items: readonly WorkspaceView[]): WorkspaceListState => ({
-  items, state: 'idle', phase: 'ready', error: null, baselinesReady: true,
+  items, archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null, baselinesReady: true,
   recentWorkspaceId: items[0]?.workspaceId,
 })
 function anchor(): { current: HTMLElement } {