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_

Inizia confrontando il flusso interattivo di `agentcore create` con i percorsi dei comandi non interattivi `agentcore create` e `agentcore add harness`, concentrandoti sui flag di memory esistenti. Verifica che create accetti le opzioni managed-memory e tuning, scriva il `harness.json` previsto, mantenga memory disabilitata per impostazione predefinita ed esponga le opzioni in `--help` e nella documentazione dei comandi.

Scritto dal modello di indicizzazione a partire dal testo della issue.