feat: select resource region during context setup

[youngjinjung-linq]

Problem

Multi-region contexts now allow one AWS identity to browse several resource regions, but unic context setup always exports the configured default region. Users cannot choose which configured region should become AWS_REGION for the shell session being set up.

Proposed behavior

• After context/account/role resolution, show a resource-region picker when the resolved context has multiple configured regions.
• Skip the picker for single-region contexts.
• Preselect the configured default region.
• Export the selected value as AWS_REGION and AWS_DEFAULT_REGION without changing the persisted default region.
• Preserve the full region list on generated SSO contexts.
• If selection is cancelled, do not change the current context or emit shell exports.

Expected flow

SSO base context -> Account -> Role -> Resource Region -> shell exports
Concrete context -> Resource Region -> shell exports

Acceptance criteria

• Works for SSO, credential, console-login, assume-role, and default contexts.
• Existing single-region setup flow remains unchanged.
• Region selection is covered by picker, setup-flow, cancellation, and export tests.
• README configuration/usage documentation reflects the setup-time selection.

[youngjinjung-linq]

@Unic-bot: assign me

[github-actions]

@youngjinjung-linq has been assigned to this issue.