Stale background TaskOutput lookups fail after /clear

What happened: After running /clear, the transcript showed background task lookups failing:

• Task Output por-git_api → "Error: No task found with ID: por-git_api"
• Task Output 3a3ebc → "Error: No task found with ID: 3a3ebc"
• followed by "Invalid tool parameters"

Repro: Spawn a background task/skill/agent, then run /clear. A subsequent
TaskOutput-style lookup against the pre-clear task ID fails to resolve, and a
follow-up tool call reports invalid parameters.

Expected: Either the stale task references shouldn't surface after /clear,
or the lookup/retry should fail gracefully instead of producing a malformed
follow-up tool call.

Environment: Windows 11, Claude Code CLI, working in a git repo.