Add local candidate validation

docs/internal/overview.md

@@ -14,6 +14,9 @@ Implemented internal packages:
   direction. Run it with `go run ./internal/apicheck` from the repository root.
 - `internal/process` runs cancellable child commands with per-child environment
   overlays.
+- `internal/candidate` captures and validates one clean local candidate, checks
+  its committed release note and local tag state, runs validation once, and
+  rechecks mutable repository state.
 - `cmd/releasekit/internal/cli` parses implemented commands, loads strict JSON
   profiles, and adapts configured checks to the process runner.