[Feature] Subagent's TaskOutput should return final result text, not full JSONL log

Problem

When a subagent uses TaskOutput tool to retrieve another background agent's result, it returns the entire JSONL conversation log instead of just the final result text.

Current Behavior

Parent Agent → launches Background Subagent A
Parent Agent → launches Subagent B to consume A's result via TaskOutput
Subagent B receives: Full JSONL log (all messages, tool calls, intermediate steps)

Expected Behavior

Subagent B receives: Final assistant message text only

Workaround

tail -1 /tmp/claude/.../tasks/{task_id}.output | jq -r '.message.content[0].text'

Why This Matters

• Subagents only need the final result to continue their work
• Full JSONL log wastes context tokens and causes confusion
• May hit token limits with large conversation logs (#15095)