Add project-name option to preview create

[tejaskash]

Summary

Adds an optional --project-name flag to agentcore create on preview so the project/folder name can be set independently from the resource name supplied by --name.

On preview, default create produces a harness. With this change, --name identifies the harness resource while --project-name controls the generated project name and directory. Existing calls without --project-name keep the prior behavior.

Validation

• npm run build
• Created a real preview harness project with: node dist/cli/index.mjs create --project-name CliProjPrevFinal --name CliHarnessResourceFinal123456 --skip-git --skip-install --json --output-dir /tmp/agentcore-cli-create-check-preview-final
• Verified agentcore.json has project CliProjPrevFinal, harness CliHarnessResourceFinal123456, path app/CliHarnessResourceFinal123456, and the harness config exists.