- Add spawn settings (maxSpawnedAgentsPerParent, maxSpawnedAgentsGlobal) to ProjectSettings type
- Implement AgentStore access and spawned agent tracking in TaskExecutor
- Add spawn_agent tool with child agent execution in isolated worktrees
- Wire up child termination on parent session end and system prompt generation
- Add comprehensive test suite (554 lines) covering spawning, limits, termination, and error handling
- Document agent spawning architecture, IPC protocol, and usage in AGENTS.md
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