feat(vendor): select command — manage include/exclude selection after initial add

vendor select <name> --add/--remove/--mode all changes what a source
contributes without advancing the pinned rev: ids are validated against
the pinned upstream (typos abort listing available ids), collisions
re-run, renames from the lockfile are honored, and the external area is
rewritten via the same staging-swap as update. include mode grows and
shrinks the include list; all mode --remove moves ids onto exclude;
--mode all resets to everything. Failed selects mutate nothing.

Closes beads: agent-lib-cd3

internal/cli/vendor.go

@@ -15,6 +15,7 @@ func newVendorCmd() *cobra.Command {
 		Short: "Curate external sources into this work repository",
 	}
 	cmd.AddCommand(newVendorAddCmd())
+	cmd.AddCommand(newVendorSelectCmd())
 	cmd.AddCommand(newVendorListCmd())
 	cmd.AddCommand(newVendorInspectCmd())
 	cmd.AddCommand(newVendorUpdateCmd())
@@ -71,3 +72,39 @@ func splitList(in []string) []string {
 	}
 	return out
 }
+
+func newVendorSelectCmd() *cobra.Command {
+	var add, remove []string
+	var mode string
+	cmd := &cobra.Command{
+		Use:   "select <name>",
+		Short: "Change a source's selection without advancing the pinned revision",
+		Long: "Manages include/exclude selection after the initial vendor add.\n" +
+			"Include mode: --add appends ids (validated against the pinned upstream),\n" +
+			"--remove drops ids and deletes their folders. All mode: --remove moves ids\n" +
+			"onto the exclude list; --add is an error (everything is selected already).\n" +
+			"--mode all resets to everything: include/exclude lists are cleared and the\n" +
+			"full pinned upstream is re-materialized. Collisions re-run on every change;\n" +
+			"renames from the lockfile are honored. The revision never moves — that is\n" +
+			"vendor update's job.",
+		Args: cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if mode != "" && mode != "all" {
+				return fmt.Errorf("--mode accepts only \"all\" (got %q)", mode)
+			}
+			opts := vendor.SelectOptions{
+				Add:     splitList(add),
+				Remove:  splitList(remove),
+				ModeAll: mode == "all",
+			}
+			if _, err := vendor.Select(".", args[0], opts, cmd.OutOrStdout()); err != nil {
+				return fmt.Errorf("vendor select: %w", err)
+			}
+			return nil
+		},
+	}
+	cmd.Flags().StringSliceVar(&add, "add", nil, "comma-separated upstream ids to add to the selection")
+	cmd.Flags().StringSliceVar(&remove, "remove", nil, "comma-separated upstream ids to remove from the selection")
+	cmd.Flags().StringVar(&mode, "mode", "", "switch selection mode (only \"all\": clear include/exclude, take everything)")
+	return cmd
+}