fix(#141): reject non-regular files in validateDocmapPath

Add IsRegular() check after Lstat so directories, FIFOs, and device nodes
produce a clear error ("docmap must be a regular file") instead of a
confusing downstream parse error.

Test: TestValidateDocmapPath_NonRegularFile

Addresses MINOR finding in review #4175.

cmd/review-bot/validatedocmap.go

@@ -61,6 +61,13 @@ func validateDocmapPath(localPath, resolvedRoot string) error {
 		return fmt.Errorf("symlinks are not allowed")
 	}
 
+	// Reject anything that is not a regular file (directories, FIFOs, device
+	// nodes, etc.) — ParseDocMapConfig expects a plain YAML file and would
+	// produce a confusing error on non-regular entries.
+	if !fi.Mode().IsRegular() {
+		return fmt.Errorf("docmap must be a regular file")
+	}
+
 	// Confine to resolvedRoot: use the fully-resolved path so that a directory
 	// symlink inside the repo cannot carry the path outside the root.
 	rel, err := filepath.Rel(resolvedRoot, resolvedPath)