Problem

PR #1428 added OpenTelemetry trace correlation at the spawner level (parent activity linked to parent session's trace), but sub-agent internal operations don't create their own child activities.

Current Behavior

• SubAgentSpawner creates an Activity("netclaw.subagent.spawn") linked to the parent session's trace
• SubAgentActor.InvokeLlmAsync — no Activity created for LLM calls
• SubAgentActor.ExecuteToolsAsync — no Activity created for tool executions

In Seq/Grafana traces, the sub-agent appears as a single flat span with no sub-spans for the work it does.

Root Cause

SubAgentActor.InvokeLlmAsync and SubAgentActor.ExecuteToolsAsync don't create child activities or set their own parent trace context. They log directly without pushing a diagnostic scope.

Proposed Fix

In SubAgentActor:

1. Create child Activities in InvokeLlmAsync:

2. Create child Activities in ExecuteToolsAsync:

3. Alternatively, use Activity.CreateRootActivity for independent traces if not wanting to inherit the parent.

Impact After Fix

Sub-agent traces in Grafana/Seq will show:

• Parent session → sub-agent spawn span → per-LLM-call spans → per-tool spans
• Total time breakdown: how much time spent in LLM vs tools
• Failed tool calls appear as error spans for easy filtering

References

• PR fix(subagents): session-correlated sub-agent log observability #1428 (parent) — added spawner-level trace correlation
• SubAgentActor.cs — InvokeLlmAsync, ExecuteToolsAsync
• SubAgentSpawner.cs — spawner Activity creation (existing pattern)