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:

docs/roadmap/remediation.md

@@ -1,6 +1,6 @@
 # Shared validation foundation remediation
 
-Status: Remediation in progress; findings 1–3 addressed, findings 4–6 pending.
+Status: Remediation in progress; findings 1–4 addressed, findings 5–6 pending.
 
 Reviewed committed implementation `f53354b`, covering the eight foundation
 commits after `0c7129a`, against the [implementation plan](implementation.md),
@@ -130,6 +130,15 @@ reads from fully populated candidates.
 
 ## 4. P2: Public and CLI error presentation loses diagnostic information
 
+**Resolution:** Addressed. Public errors use the internal diagnostic formatter
+with explicit conversions, including findings, targets, directives,
+measurements, and causes. Matching converted diagnostics reuse their contextual
+cause's rendering once. The CLI preserves outer wrappers, and callback errors
+retain their public identity. Regression tests cover both candidate operations
+and caller-created errors with and without causes.
+`go test . ./cmd/releasekit ./internal/validation ./internal/apicheck` and
+`go run ./internal/apicheck` passed.
+
 **Owners:** [public diagnostics](../../validation_error.go),
 `ValidationError.Error` and its formatter; [CLI presentation](../../cmd/releasekit/main.go),
 `candidateFailureText`; [internal formatting](../../internal/validation/diagnostic.go).