fix(actor): read actor_ref from NodeDefinition field in _detect_subgraph_cycles

Fix cross-actor subgraph cycle detection in the actor compiler by reading
actor_ref from the top-level NodeDefinition field instead of the config dict.

The _detect_subgraph_cycles(), _map_node(), and compile_actor() functions
were all reading node.config.get("actor_ref", "") instead of node.actor_ref.
Because actor_ref is a typed, validated Pydantic field on NodeDefinition (not
a key inside the untyped config dict), the old code always returned an empty
string, causing cross-actor cycle detection to silently fail and leaving the
system vulnerable to infinite recursion at runtime.

Changes:
- Fix all three call sites in src/cleveragents/actor/compiler.py
- Add Behave regression tests (features/actor_subgraph_cycle_detection.feature)
  with @tdd_issue and @tdd_issue_1431 tags on all scenarios
- Add Robot Framework integration test (robot/actor_compiler.robot)
- Fix existing test helpers to use actor_ref as top-level field
- Add CHANGELOG entry

ISSUES CLOSED: #1431

robot/helper_actor_compiler.py

@@ -66,6 +66,82 @@ def main() -> int:
             print(f"actor-compiler-fail: {exc}")
             return 1
 
+    if command == "cycle-detect":
+        # Test cross-actor cycle detection using actor_ref top-level field.
+        # Creates two mutually-referencing actors and verifies SubgraphCycleError
+        # is raised, confirming the fix for issue #1431.
+        from cleveragents.actor.compiler import SubgraphCycleError
+        from cleveragents.actor.schema import (
+            ActorType,
+            EdgeDefinition,
+            NodeDefinition,
+            NodeType,
+            RouteDefinition,
+        )
+
+        def _make_actor(
+            name: str, subgraph_ref: str | None = None
+        ) -> ActorConfigSchema:
+            if subgraph_ref is not None:
+                nodes = [
+                    NodeDefinition(
+                        id="start",
+                        type=NodeType.AGENT,
+                        name="Start",
+                        description="Start",
+                        config={"agent": "default"},
+                    ),
+                    NodeDefinition(
+                        id="sub",
+                        type=NodeType.SUBGRAPH,
+                        name="Sub",
+                        description="Subgraph",
+                        config={},
+                        actor_ref=subgraph_ref,
+                    ),
+                ]
+                edges = [EdgeDefinition(from_node="start", to_node="sub")]
+                entry, exits = "start", ["sub"]
+            else:
+                nodes = [
+                    NodeDefinition(
+                        id="leaf",
+                        type=NodeType.AGENT,
+                        name="Leaf",
+                        description="Leaf",
+                        config={"agent": "default"},
+                    )
+                ]
+                edges = []
+                entry, exits = "leaf", ["leaf"]
+            route = RouteDefinition(
+                nodes=nodes, edges=edges, entry_node=entry, exit_nodes=exits
+            )
+            return ActorConfigSchema(
+                name=name,
+                type=ActorType.GRAPH,
+                description=f"Test actor {name}",
+                model="gpt-4",
+                route=route,
+            )
+
+        actor_a = _make_actor("test/actor-a", subgraph_ref="test/actor-b")
+        actor_b = _make_actor("test/actor-b", subgraph_ref="test/actor-a")
+        registry = {"test/actor-a": actor_a, "test/actor-b": actor_b}
+
+        try:
+            compile_actor(actor_a, actor_resolver=registry.get)
+            print(
+                "actor-compiler-cycle-not-detected: FAIL — expected SubgraphCycleError"
+            )
+            return 1
+        except SubgraphCycleError as exc:
+            print(f"actor-compiler-cycle-detected: {exc}")
+            return 0
+        except Exception as exc:
+            print(f"actor-compiler-unexpected-error: {exc}")
+            return 1
+
     print(f"Unknown command: {command}")
     return 1