Persist subagent token usage to session JSONL files

Description

When using the Task tool to spawn subagents, the token usage from those subagent API calls is tracked in-memory and displayed by the /cost command, but it's not persisted to the session JSONL files.

Current behavior

• Main session messages include full usage data in assistant messages
• Subagent (Task tool) results only store the text response, not token usage
• /cost command shows accurate totals (main + subagents) from in-memory state
• Session files only contain ~50-70% of actual usage when subagents are used

Requested behavior

Include subagent usage in the tool_result or a separate message type when a Task completes:

{
  "type": "tool_result",
  "tool_use_id": "toolu_xxx",
  "content": "Agent completed...",
  "subagent_usage": {
    "model": "claude-haiku-4-5-20251001",
    "input_tokens": 97300,
    "output_tokens": 3800,
    "cache_read_input_tokens": 0,
    "cache_creation_input_tokens": 27900
  }
}

Use case

• Teams using Claude Code with AWS Bedrock need to track and report usage across multiple users
• Session files are the only persistent record of usage, but they're incomplete
• Without this data, cost tracking/auditing requires manual /cost logging or AWS CloudWatch access

Workaround

Currently using /cost output manually or AWS CloudWatch metrics, but neither integrates well with local session history.