refactor(v0.3): replace ListOpts.Projects bool with tri-state Type filter

ListOpts now exposes a Type string field (TypeAny / TypeTask /
TypeProject). TypeAny is the new way to express "both tasks and
projects" in a single ListWorkspaces call -- which the next two
commits will use to consolidate cmd/last and cmd/delete onto a
single helper, and to make 'ctask list' default to showing both
types.

Invalid Type values now return an explicit error from
ListWorkspaces (defensive against typos in callers).

cmd/list, cmd/last, and cmd/delete are migrated to the new field.
External behavior is unchanged in this commit; the cleanup of
ctask list semantics happens in a follow-up commit so the diff
stays reviewable.

cmd/delete.go

@@ -71,17 +71,12 @@ func runDelete(cmd *cobra.Command, args []string) error {
 	fmt.Printf("  Files: %d (%s)\n", fileCount, formatSize(totalSize))
 
 	// Check if this is the most recently updated workspace.
-	// v0.3: union tasks and projects so the "most recent" check spans both types.
-	tasks, _ := workspace.ListWorkspaces(root, workspace.ListOpts{
+	// Spans both tasks and projects via Type=TypeAny.
+	results, _ := workspace.ListWorkspaces(root, workspace.ListOpts{
 		IncludeArchived: false,
 		Limit:           0,
+		Type:            workspace.TypeAny,
 	})
-	projects, _ := workspace.ListWorkspaces(root, workspace.ListOpts{
-		IncludeArchived: false,
-		Projects:        true,
-		Limit:           0,
-	})
-	results := append(tasks, projects...)
 	if len(results) > 0 {
 		best := results[0]
 		for _, r := range results[1:] {