Plan filesystem sources and underlays

README.md

@@ -41,6 +41,21 @@ load once, inspect where final values came from, and distinguish explicit values
 from omissions with minimal configuration bookkeeping. Applications retain their
 domain rules and execution responsibilities.
 
+Loading is synchronous and complete on return. Applications normally load once
+near startup and then use the final configuration and metadata; ConfigKit stays
+inactive until explicitly called again. Applications may initiate additional
+loads as needed. See the [loading lifecycle](docs/config.md#completion-and-resource-lifetime)
+for completion and ownership details.
+
+Within a configuration tier, independent contributions compose strictly: shared
+mappings combine, while overlapping non-mapping definitions are errors. Between
+precedence tiers, mappings combine recursively, retaining omitted lower-priority
+children; other values replace lower-priority values. Today, the main document
+and its imports form the strictly composed base, with a selected overlay and
+explicit CLI overrides above it and typed defaults below it. The
+[configuration contract](docs/config.md#strict-document-composition) owns the
+detailed combination rules and input-specific constraints.
+
 The [architecture goals](docs/policy/architecture.md#success-criteria) explain
 these priorities and how genuine new needs can justify extending the shared
 model. The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions.