[dotnet-port-api] BackgroundTaskCompletion loop evaluator is blocked on a missing subagent/background-agents harness #1164

While porting the loop evaluators (see #1162, #1163), I found that BackgroundTaskCompletionLoopEvaluator cannot be ported yet — its entire dependency does not exist in the Go SDK.

The blocker

The .NET BackgroundTaskCompletionLoopEvaluator keeps re-invoking the agent until a BackgroundAgentsProvider reports no still-running background tasks, formatting the running BackgroundTaskInfo list ({incomplete_tasks} / {incomplete_task_count}) into the feedback.

Go has none of that layer:

• No BackgroundAgentsProvider, BackgroundTaskInfo, or any subagent/background-task harness (a grep for backgroundagent/subagent across agent/, tool/, workflow/ returns nothing).
• The parity doc (docs/dotnet-go-sdk-feature-comparison.md) confirms it: the harness row lists "subagent harness utilities" among what Go "still lacks," and the top-level gaps list includes "subagents."

So the evaluator is a thin consumer of a subsystem that first has to exist.

Proposed sequencing

1. Design + port the subagent / background-agents harness (a BackgroundAgentsProvider-equivalent context provider that spawns and tracks background agent runs, plus a BackgroundTaskInfo-equivalent). This is a sizeable owner-play feature and deserves its own design discussion — happy to draft an architecture proposal (Mermaid + API sketch) if there is interest.
2. Once that exists, BackgroundTaskCompletionLoopEvaluator is a small follow-up (same shape as the AI-judge/todo evaluators), subject to the same placement question raised in [dotnet-port-api] Port TodoCompletion and BackgroundTaskCompletion loop evaluators (placement decision) #1163.

Flagging rather than implementing, since building the whole subagent subsystem to satisfy one evaluator would be a large, design-first change — not a drive-by PR.