feat: implement SessionManager for Strands TS integration

**Summary**

The AgentCore CLI's TypeScript Strands template needs a session manager that wires Strands TS Storage / SessionManager interface to the AgentCore Memory service.

**Context**

Python's AgentCoreMemorySessionManager reads/writes session events to the AgentCore Memory service, providing durable short-term memory that survives cold starts. Strands TS now has a SessionManager plugin with pluggable Storage backends (InMemoryStorage, FileStorage, S3Storage), but no AgentCore Memory-backed implementation exists yet.

**Acceptance criteria**

- [ ] A service-backed session manager is exported from the package (e.g. AgentCoreMemorySessionManager)
- [ ] It integrates with Strands TS Agent via the sessionManager constructor option
- [ ] Conversation history is persisted to and restored from the AgentCore Memory service
- [ ] Multi-turn recall works across cold starts (agent restarts pick up prior conversation)
- [ ] Works with both STM-only (no strategies) and LTM + STM (with retrieval config) Memory resources

**Related**
- Strands TS SessionManager: https://github.com/strands-agents/harness-sdk/blob/main/strands-ts/src/session/session-manager.ts
- CLI tracking issue:

Start by reading the Strands TS SessionManager interface at strands-ts/src/session/session-manager.ts and compare it with the Python AgentCoreMemorySessionManager behavior described in the issue. Then trace the TypeScript Strands template entry point and determine how its agent receives the sessionManager option. Done means an exported service-backed manager persists and restores multi-turn history for both STM-only and LTM + STM resources.

Written by the indexing model from the issue text.