docs: add WhatsApp message flow architecture documentation
Comprehensive documentation covering:
- Complete message lifecycle from reception to delivery
- Layer-by-layer breakdown (Baileys, Monitor, Router, Agent, Dispatcher)
- Sequence diagrams for message flow, group gating, tool execution
- Key data structures (WebInboundMsg, ResolvedAgentRoute, MsgContext)
- Configuration examples for routing, group gating, media understanding
- Error handling and observability patterns
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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