agentcore init command + portability gaps: aws-targets.json gitignore guidance + CDK re-scaffold support

### Description

I'm not finding an easy way to create a project using agentcore toolkit cli and making it portable. I've found a few gaps such as:

* `aws-targets.json` hardcodes account + region — if you clone the repo elsewhere without it, `agentcore deploy` fails. But the file is environment-specific so it shouldn't be committed with real values.
* `agentcore/cdk/` is auto-generated — if missing, deploy fails ("no CDK project"), but if you run `agentcore create` to regenerate it, it refuses because the directory isn't empty.
There's no `agentcore init` that re-scaffolds just the infrastructure around existing code/config.

It would be very useful to have a way to have portable `agentcore.json` files and a mechanism to recreate the CDK project from scratch. This can also help with updating CDK to newer versions when `agentcore` cli is updated.

### Acceptance Criteria

- An `agentcore.json` created to run in an AWS account can be deployed in a new account with just one command: `agentcore deploy`

### Additional Context

_No response_

Start by reading the behavior around agentcore.json, aws-targets.json, and the agentcore/cdk/ directory, then trace the agentcore deploy and agentcore create commands. Clarify how re-scaffolding should handle existing code and configuration. Done means the documented acceptance criterion is met: an agentcore.json from one AWS account can deploy in a new account with one agentcore deploy command.

Written by the indexing model from the issue text.