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/prompts.rs

@@ -215,7 +215,7 @@ pub(in crate::config) const DEFAULT_TEAMMATE_INSTRUCTIONS: &str = indoc! {"
     ## Teammate Messaging
 
     You have tools to communicate with other agents running alongside you:
-        - `agent__send_message --id <agent_id> --message \"...\"`: Send a message to a child or sibling agent by its id, or to your parent with the reserved id `parent`.
+        - `agent__send_message --id <agent_id> --message \"...\"`: Send a message to a child, sibling, or concurrent teammate agent by its id, or to your parent with the reserved id `parent`.
         - `agent__check_inbox`: Check for messages sent to you by other agents.
 
     If you are working alongside other agents (e.g. reviewing different files, exploring different areas):