refactor: move interactive prompt helpers to a command-agnostic package

## What

The interactive stack/component selection helpers (`PromptForStack`, `PromptForComponent`) currently live in `cmd/terraform/shared/`. These include flag persistence logic that writes the selected value back to the Cobra flag set so PostRunE hooks can read it.

This placement means other command types (helmfile, ansible, packer) cannot reuse the interactive selection + flag persistence without importing from `cmd/terraform/shared/` — a wrong dependency direction.

## Why

As we add interactive selection to more command types, the prompt helpers and their flag persistence behavior should be available from a command-agnostic location (e.g., `pkg/flags/` or a new `cmd/shared/`).

## Current state

- `cmd/terraform/shared/prompt.go` — `PromptForStack`, `PromptForComponent`, `HandlePromptError`, `ValidateStackExists`
- Only terraform commands use these today
- Flag persistence was centralized into `PromptForStack` in PR #2520

## Proposed

Move the prompt helpers to a command-agnostic package so helmfile, ansible, packer, and future command types can share the same interactive selection + flag write-back behavior.

## References

- PR #2520 — centralized flag persistence into `PromptForStack`
- Review discussion: https://github.com/cloudposse/atmos/pull/2520#discussion_r2111226810