Add session management documentation guide

[kaleko]

Adds docs/SESSION_MANAGEMENT.md covering:

• Architecture approaches comparison (DynamoDB+APIGW, localStorage, AgentCore Memory listing, S3)
• Full DynamoDB + API Gateway implementation: table schema, CDK code, Lambda handler, frontend service
• Session flow (create, list, resume, update, delete)
• Session resumption via AgentCore Memory's runtimeSessionId
• Advanced patterns for long-running agents (status tracking, polling, cancellation)
• Session naming strategies
• Cost considerations
• Client-side localStorage alternative for prototypes