Expose Markdown validation selection

docs/config.md

@@ -46,6 +46,26 @@ change under formatting. It never writes formatted bytes back. Omitted or false
 disables the check. It runs after tests, selected full vet, and selected package
 compilation, before configured product checks.
 
+When `validation.markdown` is an object, ReleaseKit checks local destinations
+and supported heading fragments in committed regular Markdown files. Omission
+or `null` disables the check; `{}` enables it. Schemed and protocol-relative
+links are skipped, URI query components do not affect path checks, and other
+existing files or directories receive existence checks only. Missing local
+destinations, symlink cycles, and missing Markdown headings fail with the
+`markdown-links` identity. Outside-checkout destinations are reported as out
+of scope by default. Set
+`validation.markdown.enforce_checkout_boundary` to `true` to fail those links;
+submodule contents remain out of scope under either setting.
+
+The parser recognizes inline links, images, reference links, and CommonMark
+autolinks; link-like text in code is ignored, and raw HTML destinations are
+outside this check. Relative paths resolve from the source document in the
+captured commit. Query components are ignored for file existence, and percent
+escapes in local paths and fragments are decoded once. Fragments are compared
+in lowercase with anchors derived from visible ATX or Setext heading text.
+Anchors keep Unicode letters and numbers, underscores, and hyphens; spaces
+become hyphens, and repeated headings receive numeric suffixes.
+
 When `validation.module_distribution.workspace_files` is true, it reports
 committed blob entries whose basename is `go.work` or `go.work.sum`, including
 nested paths and symlinks. It checks candidate distribution contents regardless
@@ -71,6 +91,7 @@ For example:
       "compile_targets": ["linux/amd64", "linux/arm64"]
     },
     "go_format": true,
+    "markdown": {},
     "module_distribution": {
       "workspace_files": true,
       "local_replacements": true
@@ -130,6 +151,10 @@ preserve their causes.
 When `validation.file_size` is selected, shared validation checks committed
 blob sizes after module-distribution checks and reports the first oversized
 path with the `file-size` identity and measured and limit byte counts.
+When `validation.markdown` is selected, Markdown checking follows file-size
+inspection and runs before configured product checks. Out-of-scope observations
+are written to the command's standard error and do not fail validation unless
+the checkout boundary is enforced. Submodule observations remain nonfailures.
 Configured `checks` commands run afterward, once each in order, as additional
 product checks. Artifact builds selected by the separate `build` setting run
 after shared validation and product checks.