feat: shared wizard abstraction for TUI flows

### Problem
Each TUI wizard flow is implemented from scratch, which introduces opportunities for inconsistent behavior and larger surface area for bugs. As an example see:
https://github.com/aws/agentcore-cli/pull/1575/changes#r3433276727

In this case, we're controlling the back behavior of step of the flow individually, which led to a bug.

### Definition of Done
- Propose a general wizard abstraction for these flows (to be used in add flows, and create, etc.).
- Migrate existing consumers to a shared framework.

The framework should support all functionality we currently need for flows, and be flexible to future extensions.

Begin by comparing the existing TUI wizard flows, especially the add and create flows, and review how each currently handles step navigation and back behavior. Propose a shared abstraction that supports their current functionality, then migrate the existing consumers and verify consistent behavior across the flows.

Written by the indexing model from the issue text.