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/config/mod.rs

@@ -25,6 +25,7 @@ mod update;
 
 #[cfg(test)]
 pub(crate) use self::agent::AgentConfig;
+pub(crate) use self::agent::default_max_agent_depth;
 pub use self::agent::{
     Agent, AgentVariable, AgentVariables, complete_agent_variables, list_agents,
     list_agents_with_descriptions,