Add architecture documentation

• Detail provider abstraction layer supporting Anthropic, OpenAI, Google, xAI, and AWS Bedrock
• Explain data models: Providers, Conversations, Messages, Tasks, Blocks
• Document Task Reconciler reconciliation loop orchestrating user input, model invocations, and tool execution
• Describe CodeAct JavaScript-based tool calling system with Sobek VM sandboxing
• Cover multi-agent architecture with cost optimization and specialization benefits

Co-authored-by: construct-agent <noreply@construct.sh>

Author: Furisto