topdata: skip .md doc files in asset validation and packaging

Docs like AGENTS.md/CLAUDE.md under assets/ broke sow-topdata builds
with "unsupported topdata asset HAK resource extension". Skip .md in
both the validator and HAK package walker.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/topdata/topdata.go

@@ -2233,6 +2233,9 @@ func validateTopPackageAssets(sourceDir, dataDir string, report *ValidationRepor
 		if strings.HasPrefix(filepath.Base(path), ".") {
 			return nil
 		}
+		if strings.EqualFold(filepath.Ext(path), ".md") {
+			return nil // docs (AGENTS.md, README.md, ...) are never HAK resources
+		}
 		base := strings.ToLower(strings.TrimSuffix(filepath.Base(path), filepath.Ext(path)))
 		ext := strings.TrimPrefix(strings.ToLower(filepath.Ext(path)), ".")
 		if _, ok := erf.HAKResourceTypeForExtension(ext); !ok {