Refactor configuration handling to use directory-per-resource layout

- Introduced `_write_castle_config` function to scatter nested config into separate YAML files for programs, services, and jobs.
- Updated `castle_root` fixture to utilize the new config writing method.
- Added tests for configuration aggregation and scattering in `test_health.py`.
- Removed legacy unit handling from `manifest.py` and refactored related code in `config.py`.
- Updated documentation to reflect new configuration directory structure.
- Removed obsolete unit tests related to unit expansion.

cli/src/castle_cli/manifest.py

@@ -28,6 +28,4 @@ from castle_core.manifest import (  # noqa: F401 — explicit re-exports for typ
     ServiceSpec,
     SystemdSpec,
     TLSMode,
-    UnitKind,
-    UnitSpec,
 )