test: refactor test structure and remove mock dependencies (#12)

- Refactor integration tests to use live API calls instead of mocks
- Add comprehensive Oh My Posh integration tests with environment variable validation
- Add new test_profile_logic.bats for PowerShell profile testing
- Update test_helper.bash with improved utilities
- Remove obsolete mock JSON files (dotnet_releases.json, dotnet_releases_index.json, powershell_release.json)
- Remove obsolete scripts/test-ohmyposh-build.sh
- Remove .augment/env/setup.sh (no longer needed)
- Simplify test structure: offline unit tests + live integration tests
- Update integration tests to reference new blue-psl-10k.omp.json theme file

Author: jmcombs