Launch an autonomous subagent to perform tasks independently. Best combined with batch.

Subagent types (set via `subagent_type`):
- `research` (default): Read-only tools. For codebase exploration or gathering context.
- `general`: Full tool access. For delegating implementation work.

Notes:
1. Launch multiple tasks concurrently when possible.
2. The agent's result is not visible to the user. Summarize it in your response.
3. Each invocation starts fresh - inline any needed context into the prompt.
4. Tell it to return concise summaries with file:line refs, not full file contents.
