Add local candidate validation

docs/consumers/check.md

@@ -0,0 +1,34 @@
+# Local candidate check
+
+Use `releasekit.Check` to verify a candidate against the current local
+repository state:
+
+```go
+result, err := releasekit.Check(ctx, releasekit.ReleaseRequest{
+	RepositoryDir: "/work/project",
+	Product:       "Example",
+	Version:       "v1.2.3",
+	Validate: func(ctx context.Context, candidate releasekit.Candidate) error {
+		return runProjectChecks(ctx, candidate.RepositoryDir)
+	},
+})
+if err != nil {
+	return err
+}
+fmt.Println(result.Version, result.Commit)
+```
+
+The repository must have a clean index and worktree on `main`, and the selected
+version must be stable `vMAJOR.MINOR.PATCH`. The committed
+`docs/releases/<version>.md` note must have a first Markdown H1 containing the
+product name and exact version as complete title elements. No local tag with
+that version may already exist.
+
+The callback receives the captured repository, commit, and version. ReleaseKit
+calls it once. When omitted, ReleaseKit runs the offline Go test command. The
+check revalidates the branch, commit, and cleanliness after validation.
+
+A successful check is evidence about the local candidate only. It does not
+check whether a remote branch or tag is available and does not create a tag.
+The [`check` command](../cli.md) reads the product and optional validation
+commands from `releasekit.json`.