Implement full subagent WEKA trace replay

[dbutenhof]

Complete WEKA format replay with full tool calling and branching conversation support.

Some references:

• https://github.com/kubernetes-sigs/inference-perf/blob/main/docs/weka_trace_replay.md
• feat(agentx): AgentX v1.0 (port) ai-dynamo/aiperf#1070