add --channel flag and remove AWS Amplify legacy

- Add --channel flag to setup and update commands
- Remove aws-amplify/ folder (migrated to karate-sh repo)
- Remove amplify.yml config
- Update tracker.md with latest status

The --channel flag allows selecting stable/beta without
editing config file:
  karate setup --all --channel beta
  karate update --channel beta

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ptrthomas