Preserve structured diagnostics and candidate error context

docs/internal/validation.md

@@ -21,6 +21,12 @@ for those child commands. Missing local objects produce inspection errors;
 source preparation retains its separate network-capable operations. See Git's
 [lazy-fetch controls](https://git-scm.com/docs/git) for the command mechanism.
 
+Public validation errors convert their diagnostic fields to the internal
+failure model for shared formatting. Converted failures retain their original
+contextual cause and reuse its rendering when it already contains the same
+diagnostics. The CLI presents the complete error chain, including candidate
+context around callback-supplied diagnostics.
+
 ## Adding a shared check
 
 When an optional check is accepted for implementation: