Support managed memory in non-interactive agentcore create harness flow

### Description

The interactive `agentcore create` wizard allows users to configure managed memory for a Harness. However, the non-interactive, flag-based create flow does not expose an equivalent option.

This prevents users from automating the same Harness configuration they can create interactively. The current workaround requires creating the project first and then running add harness command separately:
```
agentcore create --name MyProject --no-agent
cd MyProject
agentcore add harness --name MyHarness --memory-mode managed
```

### Acceptance Criteria

- agentcore create accepts --memory-mode managed for Harness projects
- The generated harness.json contains memory.mode: "managed"
- Managed-memory tuning flags supported by agentcore add harness are also supported by create
- Memory remains disabled when no memory option is provided
- --help and command documentation include the new options

### Additional Context

_No response_

Comienza comparando el flujo interactivo de `agentcore create` con las rutas de comandos no interactivas de `agentcore create` y `agentcore add harness`, centrándote en los flags de memory existentes. Verifica que create acepte las opciones de managed-memory y tuning, escriba la `harness.json` esperada, mantenga memory deshabilitada de forma predeterminada y exponga las opciones en `--help` y en la documentación de comandos.

Escrito por el modelo de indexación a partir del texto del issue.