Moves the `domain` and `ports` packages from `internal/core` to `internal`.
This refactoring simplifies the directory structure by elevating the core architectural concepts of domain and ports to the top level of the `internal` directory. The `core` directory is now removed as its only purpose was to house these two packages.
All import paths across the project have been updated to reflect this change.
Restructures the internal business logic from a generic `services` package to a use-case-driven design under `internal/application`.
Each primary function of the application (`app`, `instance`, `cloudlet`, `apply`) now resides in its own package. This clarifies the architecture and makes it easier to navigate and extend.
- Moved service implementations to `internal/application/<usecase>/`.
- Kept ports and domain models in `internal/core/`.
- Updated `main.go` and CLI adapters to reflect the new paths.
- Added missing `RefreshAppInstance` method to satisfy the service interface.
- Verified the change with a full build and test run.
