Add comprehensive design document with diagrams

[kiro-agent]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
_{Learn about Kiro autonomous agent}

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Summary

Creates docs/DESIGN.md - a comprehensive design document covering the project's architecture and functionality in depth, with 8 Mermaid diagrams for visual documentation.

Document Structure (1,053 lines)

Section	Content	Diagram
1. System Architecture Overview	Component overview, key configuration table	Flowchart: Client → API Gateway → Lambda → CRT clients → DynamoDB
2. Request Flow	Full request lifecycle with success/error paths	Sequence diagram with all participants
3. Component Design	Handler class, client initialization, dual-write pattern, CRT vs Netty/Apache	Class diagram: RequestHeaderWriter, clients, transports
4. Build Pipeline	Docker env, Maven profiles, native-image compilation, SAM integration	Flowchart: multi-stage build process
5. GraalVM Native Image Config	All META-INF/native-image config files explained	Flowchart: directory tree structure
6. Infrastructure as Code	SAM template resource breakdown	Flowchart: CloudFormation resource relationships
7. Deployment Architecture	Custom runtime, bootstrap script, execution environment	Flowchart: Lambda execution internals
8. Performance Characteristics	Cold start optimization, CRT benefits, load test config	CloudWatch Insights query
9. CI/CD Pipeline	GitHub Actions workflow analysis	Flowchart: workflow steps
10. Security Model	IAM, credentials, least privilege, network security	Security summary table

What was verified

• All 10 sections present and complete
• 8 Mermaid diagrams with correct syntax
• All code blocks properly balanced
• Technical details verified against source code (SDK 2.23.3, GraalVM 25.0.2, Maven 3.9.14, etc.)