Show deployment summary before executing agentcore deploy

## Description
When running `agentcore deploy` with multiple resources (agents, memory, etc.), the deployment starts immediately without showing what will be deployed. This is concerning for users who want to review changes before they go out. The current workaround (Ctrl+G to view the stack) is not discoverable.

## Category
UX Enhancement

## Acceptance Criteria
- [ ] `agentcore deploy` displays a summary of all resources that will be deployed before starting
- [ ] Summary includes resource names, types, and any changes (create/update/delete)
- [ ] User is prompted to confirm before deployment proceeds
- [ ] The summary is shown by default (not hidden behind a key combo)

## Additional Context
- Ctrl+G currently shows the stack contents, but this is not intuitive or documented prominently