AgentCore Gateway CDK construct: CloudFormation export name collides across stacks/sandboxes

The AgentCore Gateway CDK construct (used in `web/amplify/constructs/agentCoreApplication.ts`) exports a CloudFormation output with a fixed logical name (of the form `Gateway--Url`, e.g. `Gateway-default-gateway-Url`), rather than one scoped/uniqued per stack. Deploying a second sandbox stack that also creates a gateway with the same name (e.g. `default-gateway`, from `agentcore.json`) fails at the CloudFormation layer with:

```
Export with name TokenTOKEN3753-Gateway-default-gateway-Url is already exported by stack amplify-web-main-sandbox-...
```

This means only one Amplify sandbox stack in an account/region can have a gateway named `default-gateway` deployed at a time — a second developer's sandbox, or a second branch's ephemeral stack, collides with an existing one.

### Where this was found
Discovered while trying to stand up an isolated sandbox deploy of PR #178 (`import/issue-66`) for verification, alongside the already-deployed `main` sandbox stack (`amplify-web-main-sandbox-949a6d20d0`). The new stack's CDK/CloudFormation deploy failed with the export-name collision above. This is not caused by that PR (it only changes frontend code) — it's a pre-existing bug in the shared AgentCore CDK construct that surfaces whenever two stacks in the same account/region both provision a gateway of the same name.

### Suggested fix
Scope the CloudFormation export name to the stack (e.g. include `Stack.of(this).stackName` or the construct's unique node path in the export name), or avoid a fixed-name `CfnOutput`/export altogether and pass the gateway URL between stacks via a different mechanism (e.g. nested stack parameter, SSM parameter scoped per stack, or just referencing the token directly if consumer and producer are in the same stack).

Start by reading web/amplify/constructs/agentCoreApplication.ts and trace how the AgentCore Gateway CfnOutput export name is assembled from the gateway name and stack context. Confirm the behavior with two sandbox stacks using the same default-gateway name; done means both stacks deploy in the same account and region without an export-name collision.

Written by the indexing model from the issue text.