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.

graph.example.yaml

@@ -334,6 +334,16 @@ nodes:
       Research {{topic}} in depth. Existing context:
       {{context}}
     timeout: 600               # Optional wall-clock cap, seconds. Default 300.
+    teammates: false           # Optional. When true and this node runs concurrently
+                               # with other flagged agent nodes (map fan-out branches
+                               # or the same super-step), the siblings can message
+                               # each other via agent__send_message / agent__check_inbox.
+                               # Each fan-out gets its OWN registry: a `teammates` map
+                               # fan-out and a `teammates` agent node in the same
+                               # super-step frontier form two separate teams and cannot
+                               # message each other. Peers only span a single map
+                               # node's branches or a single super-step's flagged
+                               # agent nodes.
     output_schema:             # Optional. Same extraction as llm nodes
       type: object
       properties: