feat: support STM-only memory option for TypeScript Strands agents

### Description

TypeScript Strands agents currently only offer two memory options: "no memory" or "LTM + STM". The "STM only" option (available for Python Strands agents) is missing because the Strands TS SDK and Bedrock AgentCore TS SDK do not support the necessary session management primitives.

We should support STM for TS strands agent once necessary support is available in strands & TS SDK.

### Acceptance Criteria

- [ ] "STM only" appears as a selectable memory option in the TUI and is accepted via --memory shortTerm for TypeScript Strands agents
- [ ] Selecting "STM only" wires the agent to the deployed AgentCore Memory resource via the TS SDK's session manager (durable — survives cold starts)
- [ ] The LRU cache in the "no memory" template path is replaced with Strands-native SessionManager + InMemoryStorage
- [ ] Multi-turn recall works within a session (turn 2 remembers turn 1)
- [ ] Sessions are isolated (session A's history does not bleed into session B)
- [ ] Behavior matches the Python Strands template for the equivalent memory options

### Additional Context

#1636: add support for TS memory
#1666: short-term LRU cache for per-session STM
Strands TS storage interface: https://github.com/strands-agents/harness-sdk/blob/main/strands-ts/src/session/session-manager.ts
TS SDK tracking issue: https://github.com/aws/bedrock-agentcore-sdk-typescript/issues/214