claude-code: drop what was removed while away, render one at a time, refuse an empty settings set

Review of the nox-mesh plugin: a watch hands over what is there, not what
went, so the view is pruned to the state's keys before it starts; the
watches and the tools render one at a time, as the home's record is read
and written whole; the register answer names how an item is offered.

modules/claude-code/cmd/claude-code/config_tools.go

@@ -157,6 +157,9 @@ func configTools(p Paths, state ConfigState, view *ConfigView) []stdio.Tool {
 			},
 			Run: func(a map[string]any) (any, error) {
 				given, _ := a["settings"].(map[string]any)
+				if len(given) == 0 {
+					return nil, errors.New("no settings given; to remove a scope's settings, claude_code_settings_clear")
+				}
 				return SetSettings(p, scopeOf(a), nodesOf(a["nodes"]), func(held map[string]any) map[string]any {
 					if boolArg(a, "replace") {
 						return given
@@ -191,6 +194,9 @@ func configTools(p Paths, state ConfigState, view *ConfigView) []stdio.Tool {
 				return SetSettings(p, scopeOf(a), nodesOf(a["nodes"]), func(held map[string]any) map[string]any {
 					perms, _ := held["permissions"].(map[string]any)
 					rules, _ := perms[list].([]any)
+					if len(rules) == 0 {
+						return held // nothing to remove: what the scope holds stays as it is
+					}
 					kept := []any{}
 					for _, r := range rules {
 						if r != rule {