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/last.go

@@ -32,24 +32,15 @@ func runLast(cmd *cobra.Command, args []string) error {
 	root := config.ResolveRoot()
 
 	// Scan all non-archived workspaces (tasks AND projects) and find the most
-	// recently updated. v0.3: ListWorkspaces filters by type, so we union the
-	// two type buckets here so `last` keeps working for both.
-	tasks, err := workspace.ListWorkspaces(root, workspace.ListOpts{
+	// recently updated.
+	results, err := workspace.ListWorkspaces(root, workspace.ListOpts{
 		IncludeArchived: false,
 		Limit:           0,
+		Type:            workspace.TypeAny,
 	})
 	if err != nil {
 		return err
 	}
-	projects, err := workspace.ListWorkspaces(root, workspace.ListOpts{
-		IncludeArchived: false,
-		Projects:        true,
-		Limit:           0,
-	})
-	if err != nil {
-		return err
-	}
-	results := append(tasks, projects...)
 
 	if len(results) == 0 {
 		fmt.Fprintln(os.Stderr, "No active workspaces found.")