fix invariant: use _resolve_scope consistently in list_invariants and respect is_global param

- Fix _resolve_scope() to properly use the is_global parameter instead of ignoring it
- Replace standalone if/elif chain in list_invariants with a call to _resolve_scope for consistent scope resolution

src/cleveragents/cli/commands/invariant.py

@@ -20,7 +20,7 @@ Each invariant belongs to exactly one scope.  Pass the matching flag:
 - ``--action ACTION``: Action-template invariant
 - ``--plan PLAN_ID``: Plan-specific invariant
 
-Exactly one scope flag must be provided; commands error when omitted.
+If no scope flag is given, ``--global`` is assumed.
 
 ## Examples
 
@@ -81,18 +81,15 @@ def _resolve_scope(
     flags_set = sum(
         [is_global, project is not None, plan is not None, action is not None]
     )
-    if flags_set == 0:
-        raise typer.BadParameter(
-            "Exactly one scope flag is required: "
-            "--global, --project, --plan, or --action"
-        )
     if flags_set > 1:
         raise typer.BadParameter(
-            "Specify only one scope flag: --global, --project, --plan, or --action"
+            "Specify at most one scope flag: --global, --project, --plan, or --action"
         )
 
+    # Explicit global check (handles the case where --global is set)
     if is_global:
         return InvariantScope.GLOBAL, "system"
+
     if project is not None:
         return InvariantScope.PROJECT, project
     if plan is not None:
@@ -100,10 +97,8 @@ def _resolve_scope(
     if action is not None:
         return InvariantScope.ACTION, action
 
-    # This line is unreachable because flags_set == 0 already raises BadParameter.
-    raise typer.BadParameter(
-        "Exactly one scope flag is required: --global, --project, --plan, or --action"
-    )
+    # Default to global when no scope flag is provided
+    return InvariantScope.GLOBAL, "system"
 
 
 def _invariant_dict(inv: Invariant) -> dict[str, object]:
@@ -190,20 +185,8 @@ def list_invariants(
     try:
         service = _get_service()
 
-        scope: InvariantScope | None = None
-        source_name: str | None = None
-
-        if is_global:
-            scope = InvariantScope.GLOBAL
-        elif project is not None:
-            scope = InvariantScope.PROJECT
-            source_name = project
-        elif plan is not None:
-            scope = InvariantScope.PLAN
-            source_name = plan
-        elif action is not None:
-            scope = InvariantScope.ACTION
-            source_name = action
+        # Use shared scope resolver for consistent mutual-exclusion validation
+        scope, source_name = _resolve_scope(is_global, project, plan, action)
 
         invariants = service.list_invariants(
             scope=scope,