feat: teammates flag enabling peer messaging between concurrent graph agent nodes

Agent nodes marked teammates: true whose sub-agents run concurrently (map fan-out branches or multiple flagged agent nodes in one super-step) now form a team: each member gets a pre-provisioned inbox in a messaging-only PeerRegistry before any branch starts, a teammate roster injected into its prompt, peer resolution in agent__send_message, and a peers section in agent__list_running. Registries are scoped to a single fan-out and carry no lifecycle semantics: outputs still flow exclusively through collect_into/state reducers, finished peers reject new messages, and nothing registers in a supervisor. Also hardens the agent-recursion depth guard to work without a supervisor and adds validator warnings for zero-valued orchestration limits.

src/graph/types.rs

@@ -239,6 +239,9 @@ pub struct AgentNode {
 
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub timeout: Option<u64>,
+
+    #[serde(default)]
+    pub teammates: bool,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]
@@ -545,6 +548,41 @@ nodes:
         }
     }
 
+    #[test]
+    fn agent_node_teammates_parses_and_defaults_false() {
+        let yaml = r#"
+name: g
+start: a
+nodes:
+  a:
+    id: a
+    type: agent
+    agent: helper
+    prompt: hi
+    teammates: true
+    next: b
+  b:
+    id: b
+    type: agent
+    agent: helper
+    prompt: hi
+    next: e
+  e:
+    id: e
+    type: end
+    output: done
+"#;
+        let graph: Graph = serde_yaml::from_str(yaml).unwrap();
+        match &graph.get_node("a").unwrap().node_type {
+            NodeType::Agent(n) => assert!(n.teammates),
+            _ => panic!("expected Agent variant"),
+        }
+        match &graph.get_node("b").unwrap().node_type {
+            NodeType::Agent(n) => assert!(!n.teammates),
+            _ => panic!("expected Agent variant"),
+        }
+    }
+
     #[test]
     fn deserializes_mcp_tools_at_graph_and_node_level() {
         let yaml = r#"