docs: Add comprehensive WebSocket API reference
Add docs/WEBSOCKET_API.md with complete documentation for all 32 WebSocket
actions including:
- Message envelope format and error codes
- System flow diagram and typical workflow
- Board, Column, Task CRUD operations
- Task subscription for ACP streaming
- Agent management (list, launch, status, logs, stop, types)
- Orchestrator control (status, queue, start, stop, prompt, complete)
- Server notifications (acp.progress, acp.log, acp.result, etc.)
- Complete JavaScript usage example
- Quick reference table
Each action includes purpose, flow position, request/response examples,
payload field tables, and side effects.
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