This commit refactors the codebase to use dependency injection for mocking external dependencies, removing the need for in-code mocking with the `BORG_PLEXSUS` environment variable.
- Interfaces have been created for external dependencies in the `pkg/vcs`, `pkg/github`, and `pkg/pwa` packages.
- The `cmd` package has been refactored to use these interfaces, with dependencies exposed as public variables for easy mocking in tests.
- The tests in `TDD/collect_commands_test.go` have been updated to inject mock implementations of these interfaces.
- The `BORG_PLEXSUS` environment variable has been removed from the codebase.
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