feat: Coordinator Mode with SendMessageTool (Phase 2D)
- CoordinatorMode: env-var activation, system prompt, tool filtering
- SendMessageTool: direct message, broadcast, message queuing
- Coordinator allowed tools: Agent, SendMessage, TaskStop, TaskGet, TaskList, TaskOutput
- Worker allowed tools: Read, Write, Edit, Bash, Grep, Glob, etc.
- AppConfig: coordinator mode detection, specialized system prompt
- Registered SendMessageTool in ToolRegistry
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
