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_

Start by comparing the interactive `agentcore create` flow with the non-interactive `agentcore create` and `agentcore add harness` command paths, focusing on the existing memory flags. Verify that create accepts the managed-memory and tuning options, writes the expected `harness.json`, keeps memory disabled by default, and exposes the options in `--help` and command documentation.

Written by the indexing model from the issue text.