[observability] Sub-agent internal steps not traced to Langfuse — delegated runs are opaque single spans

[M3gA-Mind]

Part of #4654. Surfaced by Langfuse tracing (staging, 2026-07-07).

Problem: when the orchestrator delegates to a sub-agent (e.g. retrieve_memory, manage_profile_memory), the sub-agent's internal work is not emitted as nested Langfuse observations or a linked child trace. In the parent trace it appears as a single opaque SPAN with only the aggregate input/output; the sub-agent's own model calls and tool-call loop — the thing that actually consumed 81s and hit the tool-call limit — are invisible.

Evidence (trace thread-5e39f26f-51fa-4d8d-b813-d6b5a9572aba:b1546030-5ed4-4c82-833a-3f443be6f8bd): one retrieve_memory SPAN (81s), no child observations, no separate sub-agent trace in the same time window / session. Root-causing the incomplete run required parsing the span's text output rather than reading spans.

Fix: project sub-agent (spawned) runs into Langfuse as nested observations under the parent span (or as a linked child trace with parent/root run correlation), so each sub-agent model call + tool call is its own generation/span. This is the harness parent/root event-correlation surface (relates to the observability items in the TinyAgents audit #4632).

Impact: delegated/sub-agent behavior — the most interesting and failure-prone part — can't be observed or debugged in Langfuse.

[M3gA-Mind]

Screening note: sub-agent tracing is covered by merged #4506 (title explicitly includes 'subagents'; ModelCallCompleted carries subagent_task_id + scope). Deployed v0.58.12 predates it. Holding rather than dispatching a duplicate; will verify sub-agent nesting on a #4506-inclusive deploy before any further work.

[M3gA-Mind]

Confirmed on a live sub-agent-spawning turn (2026-07-08, batch-chunking fix #4676 let the large trace land).

Parent turn thread-409c6c0d-b757-448f-a529-04686c9bd74b54a670d-3125-48ce-b0e6-7602c9862a52 (1999 obs): every spawned sub-agent is an opaque top-level span with zero nested children — spawn_async_subagent, spawn_parallel_agents, retrieve_memory ×4. 0 of 1999 observations have a parentObservationId.

The async sub-agent's real work landed as a separate trace thread-409c6c0d-b757-448f-a529-04686c9bd74b:bgdeliver-562e4d12-… (~10 min later) — and that one does nest properly (subagent.iteration#1–5 with nested tool calls/generations). But it is not linked to the parent: its trace metadata has parent_run_id = null, root_run_id = null, run_id = null — only the shared sessionId/thread loosely correlates them.

Net: sub-agent runs ARE traced (with internal nesting), but as disconnected sibling traces with no parent/root run correlation — you can't navigate from the opaque spawn span to the sub-agent's detail. Fix: stamp parent_run_id/root_run_id onto the spawned sub-agent's Langfuse trace (from its RunContext), and/or nest the sub-agent observations under the parent's spawn span.