feat(#141): export FileCoveredByDocMap helper in review/docmap.go

Adds FileCoveredByDocMap(cfg *DocMapConfig, file string) bool — a thin wrapper
over the existing unexported mappingMatches that lets cmd/ check per-file docmap
coverage without duplicating glob logic.

Also adds unit tests covering matched globs, non-matching paths, empty file,
and empty config.

review/docmap.go

@@ -66,6 +66,18 @@ func ParseDocMapConfig(localPath string) (*DocMapConfig, error) {
 	return &cfg, nil
 }
 
+// FileCoveredByDocMap reports whether at least one paths: glob in any mapping
+// of cfg matches the given file path. It is used by static validation tooling
+// (e.g. the validate-docmap subcommand) to check per-file docmap coverage.
+func FileCoveredByDocMap(cfg *DocMapConfig, file string) bool {
+	for _, mapping := range cfg.Mappings {
+		if mappingMatches(mapping.Paths, []string{file}) {
+			return true
+		}
+	}
+	return false
+}
+
 // MatchDocs returns deduplicated doc paths for the given changed file paths.
 // A mapping matches if any of its path globs matches any of the changed files.
 func MatchDocs(cfg *DocMapConfig, changedFiles []string) []string {