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.

core/src/castle_core/manifest.py

@@ -298,58 +298,3 @@ class JobSpec(BaseModel):
 
     manage: ManageSpec | None = None
     defaults: DefaultsSpec | None = None
-
-
-# ---------------------
-# Unit spec — unified definition
-# ---------------------
-
-
-class UnitKind(str, Enum):
-    TOOL = "tool"
-    SERVICE = "service"
-    SITE = "site"
-    JOB = "job"
-
-
-class UnitSpec(BaseModel):
-    """Unified program+deployment definition.
-
-    Expands into ProgramSpec + optional ServiceSpec/JobSpec at config load time.
-    """
-
-    id: str = ""
-    kind: UnitKind
-    description: str | None = None
-
-    # Program identity
-    source: str | None = None
-    stack: str | None = None
-    system_dependencies: list[str] = Field(default_factory=list)
-    install_extras: list[str] = Field(default_factory=list)
-    version: str | None = None
-    build: BuildSpec | None = None
-    tags: list[str] = Field(default_factory=list)
-
-    # Service fields (kind=service)
-    port: int | None = Field(default=None, ge=1, le=65535)
-    path_prefix: str | None = None
-    health_path: str | None = None
-
-    # Job fields (kind=job)
-    schedule: str | None = None
-    timezone: str = "America/Los_Angeles"
-    argv: list[str] | None = None
-
-    # Simplified env (flat dict, replaces defaults.env nesting)
-    env: EnvMap = Field(default_factory=dict)
-
-    @model_validator(mode="after")
-    def _validate_kind_fields(self) -> UnitSpec:
-        if self.kind == UnitKind.SERVICE and self.port is None:
-            raise ValueError("kind=service requires 'port'")
-        if self.kind == UnitKind.JOB and self.schedule is None:
-            raise ValueError("kind=job requires 'schedule'")
-        if self.kind == UnitKind.JOB and self.argv is None:
-            raise ValueError("kind=job requires 'argv'")
-        return self