list_agents returns empty while background agents are still visibly running in UI #3514

Bug: list_agents shows no agents while tasks are still active in UI

Description

When multiple background agents are launched during a session, calling list_agents returns <no background agents> even though the user can see 7 active agents in the Copilot CLI tasks UI. The assistant incorrectly concludes "no agents running" and calls task_complete, while agents are still executing in the background.

Steps to Reproduce

1. Start a Copilot CLI session
2. During a complex review task, background agents are launched (e.g., explore agents for parallel codebase investigation)
3. After the main analysis completes, call list_agents
4. list_agents returns <no background agents>
5. Meanwhile, the user sees 7 active tasks in the UI task list

Expected Behavior

list_agents should return all currently running background agents, matching what the user sees in the UI.

Actual Behavior

list_agents returns empty (<no background agents>) while the UI shows 7 active agents still running. The assistant has no way to detect or stop these orphaned agents.

Impact

• Assistant cannot detect running agents → cannot stop them
• Assistant incorrectly reports "no agents running" to the user
• Orphaned agents consume resources silently
• User has to manually identify and stop stale agents
• Breaks the HITL trust model — assistant says one thing, UI shows another

Environment

• Copilot CLI version: 1.0.54
• Model: Claude Opus 4.6
• OS: macOS (Darwin)

Additional Context

The agents were likely launched in a previous checkpoint/compaction cycle. After context compaction, the agent IDs may have been lost from the conversation context, making them invisible to list_agents but still running in the runtime.