AWS Context Discovery

Before doing any AWS work, read the user's local AWS config. Don't guess the region, and don't ask the user for things their config already answers.

What to discover

Run these at the start of the AWS work and remember the results for the rest of the session.

1. Active profile

AWS_PROFILE env var, else default. If the user mentioned a profile in their prompt, that overrides. If the named profile doesn't exist in ~/.aws/config, surface that clearly.

2. Region