feat(TASK-008): add shared transient matcher and retry support to agent nodes

Hoist the llm node's transient-error matcher to graph::is_transient_error,
extend it with broken-pipe / connection-error / error-sending-request
signatures, and give AgentNode an opt-in max_attempts retry loop with
llm-parity semantics: contexts applied before the transient check, fresh
wall-clock budget per attempt, peer identity (assignment + registry)
re-armed before each retry, retirement exactly once after the final
attempt, and structured extraction outside the loop.

src/graph/executor.rs

@@ -827,6 +827,7 @@ mod tests {
                 state_updates: None,
                 output_schema: None,
                 timeout: None,
+                max_attempts: 1,
                 inputs: None,
                 teammates,
             }),