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.

Cargo.toml

@@ -146,6 +146,7 @@ ctor = "1.0.13"
 pretty_assertions = "1.4.0"
 rmcp = { version = "3.1.2", features = ["server"] }
 serial_test = "3"
+tokio = { version = "1.34.0", features = ["test-util"] }
 
 [[bin]]
 name = "coyote"